Wikiedia's article on the Battle of Ascaulum (sorry for bad spelling, Im on my phone and going from memory), Pyhrrus of Epirus brought some 40,000+ soldiers to the battle, and suffered 3000 casualites. How did the loss of 7% of his army prevent his capture of the city, and prompt his legendary comment (misquote, "Any more such victories and we shall be undone")?
In defeating the Romans at Haraclea and Asculum, Phyrrus lost a great deal of men, friends and commanders with no more men available to recruit and replenish his staggering losses.
Although losing less men than the Romans, the Romans had many more sources to recruit more men from and therefore received a less severe hit to their war effort.
The thing to remember about ancient battle (and for medieval battle) is that the bulk of the killing happened after one side broke and ran away - allowing their opponents to strike them down from behind.
Because of this fact, losses on the winning side are pretty much always very low in comparison to the losers. Those 3,000 casualties meant that the Romans killed a huge amount in very bitter fighting before routing.
Lastly the casualties would have been on the front ranks - and traditionally you placed your best men in the front and the second best at the very rear (to prevent people from running away). A hard fought battle means you've suffered heavy losses in your file leaders and the next fight your file closer will now be the file leader and you're going to have to find another reliable veteran soldier to close the file.
It's also worth pointing out that Epirus was tiny and ill-equipped to replace their manpower losses, while Rome even at this point held nearly the entire Italian peninsula. Rome had reserves, the Epirotes did not.