I don't really get what causes overextension myself.
In my Milan game in about 30 years i conquered most of Italy and a bunch of southern Germany including half of austria. When I stopped expanding to let my infamy cool down (it was well over the limit) I had the second highest amount of provinces in Europe yet only three of them were cored (Milan started with 5 provinces, but IIRC you have to wait 50 more years for 2 to core).
Despite this, I never got the overextension effect, which i was sure I was going to have to deal with.
Whats more, all of my first 3 or 4 slider moves were towards centralization, and it never popped up after any of them.