Classical battle sizes vs medieval battle sizes

What would you like me to explain? 
The sizes of each battle? 
If so, then medieval battles were smaller. I personally think its because countries were smaller and less unified which caused their armies to be smaller. A classical battleThey are a little bigger than the medieval battles but they had better armour and weapons. I'm not real good with history but I do stay up to date on battles whether they are Greek, Roman, Classic, or medieval battles.

The natural habitat around them