The AI tends to hide a few troops (even entire stacks) in trees, so there is always a rather large error percentage. Also, they can pump out units fairly quickly if they need to (In my Danish campaign, it took me 3 turns to march from the nearest region to Bulgar. During that time, the Russians down to their last region pumped out 3 cavalry a turn and sent strong half stacks at my advancing force) While it may be true that the other factions don't have as much military as we do, even waiting a bit longer won't cause them to build a lot more troops - they will just keep sending suicide squads.

The only way to really stop expansion is to make our enemies want to be at peace with us or become our vassals. According to the campaign_db file, this occurs when you control most of the regions adjacent to all of theirs and have a considerably large force near the border or in their lands at that time.