I wonder if the increased importance of siege warfare might have had something to do with it. Many ancient wars were decided by field battles, so it made sense to bring all your troops along to maximise your chances of winning. In the Medieval period, OTOH, the countryside was very densely fortified, meaning that even if you lost a field battle your enemies would still have to capture all seventy bajillion of your castles to win. Keeping a huge army in place long enough to win all these sieges would be a logistical nightmare in any age, so it made more sense to take along a smaller force which was big enough to carry out sieges but not so big as to place huge strains on your country's logistics.