Expanding and based off a previous answer I wrote here, numbers in the Malayan Campaign do not tell the whole story.
The Commonwealth forces in Malaya were first and foremost lacking training and equipment. Malaya Command, the Army formation tasked with defending Malaya were comprised out of units from throughout the Empire. Under them there was III Indian Corps made out of the 11th and 9th Indian Divisions, there was the 8th Australian Division and then the Singapore Fortress garrison. Of these units, the 11th and 9th Indian Divisions had their experienced NCOs and soldiers siphoned off throughout the initial years of WWII to reinforce formations in North Africa leaving the divisions with mostly fresh troops reducing the combat capabilities. The 8th Australian Division were mostly reservist and fresh troops which had just finished their training before being shipped out to Malaya as reinforcements, while they were decently trained and equipped they were still largely untested and inexperienced. Within the Indian Divisions and Singapore Fortress there were a handful of Battalions made of British regulars that were experienced and well trained but these troops were the minority of Malaya Command's forces. As of the 24th of November 1941, just a couple of days before the start of the Malayan Campaign, Malaya Command's forces only numbered 10 Brigades in total, all of which were lacking adequate anti-tank weapons and two of them were lacking artillery. Suffice to say they were not well equipped or prepared for the Japanese attack.
The Japanese on the other hand dispatched mostly veteran troops to take part in the Malayan Campaign. General Yamashita had at his disposal four divisions but actually elected to leave one of them behind in order to ease his supply lines. The Japanese forces brought to the campaign comprised of the 5th and 18th Infantry Divisions which were veteran and experienced units of the war in China. In addition the Imperial Guards Division was sent which despite its impressive name actually an untested unit that was used mostly for ceremonial duties. Nevertheless they were a long standing and well trained unit despite being untested. In addition to his units being mostly made up of fully trained veterans, the Japanese also deployed a tank battalion to Malaya which would prove to be a significant advantage as it allowed them to repeatedly breakthrough Commonwealth positions during the Campaign. Hence despite their numerical inferiority, the Japanese more than made up with it in quality.
Despite the British holding numerical superiority for the entirety of the Campaign, strategic limitations resulted in them failing to concentrate their forces and the Japanese actually held local superiority in almost every battle of the Campaign. The British, especially after losing any semblance of naval control with the sinking of the HMS Prince of Wales and Repulse, were forced to maintain troops throughout the East coast of Malaya and in Singapore to prevent a Japanese naval landing outflanking them. This resulted in the Singapore fortress troops remaining at the garrison in Singapore and the 8th Australian Division taking up positions in Johor in Southern Malaya. This left only III Indian Corps to defend the Malayan Frontier form the Japanese attack. What made matters worse was the RAF had decided before the war to unilaterally build airfields without consulting the Army which resulted in them being situated near the coast to improve aircraft range over the South China Sea. These were a liability as they were poorly situated for defense on the ground and were dispersed over all over Northern Malaya from Alor Star to Kuantan. As the British could not afford the Japanese capturing these airfields and gaining a forward base for their aircraft they were force to parcel out their troops to defend them diluting their force concentration. In effect, this resulted in the divisions of III Indian Corps being unable to provide mutual support for each other as they were geographically too far apart and essentially fought as lone divisions instead of a corps for the initial phases of the Campaign. Thus we can clearly see that the British only held numerical superior in absolute terms and that when it came to the Campaign the were enable to bring all of them to bear at once.
The British initially also had a plan, Operation Matador, which was supposed to advance into Southern Thailand to capture the port of Singora to deny it from the Japanese as this was identified to be their main landing area. The plan was never carried out owing to the political issues of a preemptive strike on a neutral country but the troops preparing to carry it out, the 11th Indian Division were kept on standby throughout the lead up to the Japanese landings. This threw the whole division off balance as they stopped work on their defensive positions at Jitra in preparation for an attack into Thailand only for it to be called off abruptly after being on standby for over two days and them being hastily ordered back to Jitra to dig in. This meant that 11th Indian Division at the start of the Campaign had not fully settled into their positions and their defensive works were incomplete allowing the Japanese vanguard from Singora to route the entire division owing to their disorganization. This started a cascade of defeats as the British were constantly kept on the backfoot and were not allowed regroup to form an effective defense by the Japanese. This led to units of III Indian Corps being constantly outflank or being forced to retreat due to breakthroughs in other parts of the line. Coupled with the disorganization throughout the formation as the Japanese were able to breakthrough and attack HQ units before they could withdraw, it resulted in III India Corps receiving a mauling for the first month of the campaign. A particularly disastrous instance of this was at the Battle of Slim River where the Japanese tanks punched through the British defense in depth along the entire road, cutting their forces in half and captured the bridge over the river intact. When General Wavell arrived in Malaya in the start of January taking over as C-in-C Far East, he quickly came to the conclusion that III Indian Corps was thoroughly ineffective as a fighting unit and ordered General Percival, GOC Malaya Command, to withdraw them behind the 8th Australian Division at Gemas and Muar in order for them to rest and refit.
This led to the Battle of Muar as the Japanese continued their drive south against the Australian positions. Defending Muar and Gemas was only one brigade of the 8th Australian Divison, the other situated at Endau and Mersing to prevent outflanking from the east coast. The Australians were, like III Indian Corps, dispersed and thus failed to achieve proper force concentration. Supporting the Australians Muar was the 45th Indian Brigade that had freshly arrived from Indian and were comprised out of raw recruits. Despite a good effort put up by the Australians at Gemas and Muar inflicting substantial casualties on the Japanese, the two brigades had no real hope of stopping the Japanese assault which had defeated a much larger formation. Eventually this culminated in the withdrawal to Singapore island where Malaya Command was reinforced by the 18th Division (not to be confused with the Japanese 18th Division) which had been rerouted from North Africa. By this point III Indian Corps was entirely ineffective as a corps level unit and its two divisions were amalgamated into a single roughly division sized unit and were in the process of reogranising. Thus they were kept in reserve while the defense of the island was left to the 8th Australian Division, the newly arrived 18th Division and a collection of fortress and independent units like the 45th Indian Infantry Brigade.
Continued