I don't have the citations at my fingertips or access to my library, but plans for a fifth Canadian division were abandoned in 1917, and the troops sent to reinforce the existing four on the continent. British brigades were further reduced from four battalions to three in 1918, but Canadian brigades remained with four battalions. I'm not certain if this reduction applied to the Australian army as well (though a quick internet search about the Australian Corps order of battle suggests that it did), but the short answer to your question is that Canadian divisions were unusually large for a British formation.
In terms of total numbers, about 640,000 Canadians served in the Canadian expeditionary force, and about 350,000 Australians in the Australian imperial force (over the duration of the conflict). These numbers are from memory, so please correct me if anyone has more accurate figures, but these are roughly the numbers we are dealing with.
The difference in the number of divisions is therefore more a consequence of organization than a disparity in the number of men who served.
Source: Tim Cook, Shock Troops
In addition to what /u/bodie87 has posted, both Canadian and Australian governments pursued a policy of military expansion that was unsustainable. Both Canada and Australia enjoyed a huge influx of volunteers when the war began. For Australia, the numbers of volunteers peaked again during the Gallipoli campaign. The large numbers of recruits allowed Australia and Canada to field five and four divisions respectively but soon ran into the problem of reinforcing those divisions as volunteer recruitment fell to a trickle. Canada got around this by introducing conscription in 1917, conscripting French-Canadians and Anglo-Canadians for service in France. The Australian government failed to introduce conscription twice, preventing them from compelling me to enlist and so was not able to provide the reinforcements necessary to maintain its divisions at full strength. Had the war continued, the five Australian divisions would have been forced to reduce the number of understrength battalions in order to reinforce other understrength battalions. As it was, Australia disbanded the 60th Battalion (other battalions were ordered to as well but refused) and reinforced the 57th Battalion.
Interestingly, New Zealand never suffered this problem. New Zealand possessed a robust system of compulsory military training prior to the war that was designed to provide an expeditionary force for service overseas if the British Empire needed it. New Zealand fielded a single division during the war and kept this divisions well supplied with reinforcements drawn from New Zealand’s territorial defence force. New Zealand never exceeded its single division commitment, cautious of finding itself in a situation similar to that of Australia or Canada. At one point during the war, the idea of raising a fourth brigade for service in France was floated and was received extremely negatively by the New Zealand public who were not willing to accept a larger commitment. This particular event happened during New Zealand’s own process to introduce conscription. It was thought that if New Zealand possessed the available manpower to field an additional brigade then why was compulsion required to reinforce the exisiting division? The notion was soon dropped and the additional manpower shifted to reinforce the division in France.