A lot does depend on the player and how he is impinging, or not, on the HRE.
When I took over Emperorship in my current game, which was around 1560, there were still 30 princes, and now there's 32 since I've released a couple for IA.
The main factor here was that I was constantly battering Austria. I fought three wars with them, all total victories. In the first two I forced them to release so many nations they were cut from 17 provinces down to 4. They later integrated Hungary which took them back up, but never regained any of the rest of their former territory.
They were doing a lot of Force Religion and stuff like that. But I've kept them pretty much on a leash and they had no ability to blob. Ditto a lot of other HRE nations. Munster for example I force-vassalised before 1500 and annexed some years later. Netherlands I have fought three wars against, trying to vassalise them to stop that awful event where my Netherlands provinces want to defect, but I never pushed it hard enough to actually vassalise them and they kept getting new territory via that same event applying to AI nations. But I forced them to release Holland once, and so both Holland and Netherlands are acting as a block on the other; neither can expand much bigger than 3-4 provinces.
Anyway, point being that if you're active in the HRE region yourself, blobbing will be much reduced - especially if you make Austria a specific target. Even if you're just European, without specifically focusing on HRE, you're going to impinge them a fair bit. I would expect that most of the times the HRE and HRE-nations grow unchecked is when the player is further away.
So if you don't like the blobbiness, go de-blob them
