I got like 110 in a wave once...
Anyway, they're not getting lazier. It's just that there's only one of you, and the more dwarves there are, the less you are able to tell them all what to do and manage them, so they finish their tasks and you aren't there to give them new ones yet. That's all. It isn't really a problem to have dwarves not doing anything, if you don't have anything you need done...
If and when you order something, though, and it is taking forever, just realize that you have spare labor, and perhaps shift more people to that skill.
Burrows are really helpful. If you have 17 miners, and they all try to dig the same spot, it is actually slower after awhile, because each one digs one block and then goes back to bed. But if you take 3 miners and put them in a burrow that includes dorms + food + Mining site A, and then another 3 dwarves in a burrow that includes dorms + food + Mining site B, etc., then you get amazing productivity.
Same for masons and mechanics. Have your really skilled ones work in a burrow with the workshops + dorms + food, where skill matters most. Have your unskilled ones burrowed everywhere except the workshops, so that the grunt masons are always the ones building the constructed walls, etc., and the grunt mechanics are the ones installing crap, and they dont sneak in and make crappy quality furniture.
If you do stuff like that, then you don't have to worry about any downsides of having 15 masons or miners, etc., and many more dwarves will be working at any given time.