I think rebel stack size (the unrest kind) is influenced by the sum total dev from provinces generating unrest for that rebel type. And they typically spawn in the province with the highest dev (of the aforementioned provinces). 11k troops coming solely out of 3 dev sounds a tad high, is that from actual gameplay?
Though I agree the numbers can sometimes get quite ridiculous -- in my run as Manchu horde, culture-converting everything (for extra banners), I had hundreds of provinces that could only spawn tribal rebels. So whenever I got overextended, the potential rebel stack could be a million strong because it takes into account every province with more than +0.1 unrest (which was nearly all of them).
Definitely stupid, though I guess they'd be much less of a threat if they'd have only base discipline. There are a number of stupid things in the game honestly, like how hiring a +5% discipline advisor takes effect
immediately (even mid-battle). So if you know a rebellion is coming up, switch to a crap military advisor, and only hire the discipline one after the monthly tick when the rebels spawn (I
think that should apply the additional 5% bonus only to your troops).
Or, if you're getting desperate (and happy with a tedious game), get used to pausing the game on the 1st of the month. It's when the rebellion ticks up (say 90% -> 100%), but the rebels only spawn on the next day after that. So you should have a chance to do very targeted harsh treatment (100% -> 70%), preventing the rebellion entirely