I have not seen different sizes. The key to keeping them on is to take a tiny drill bit and put a tiny hole in the center so it doesn't create an air pocket to keep it from seating.
At least that's what I did after mine popped off once.
Exactly.
Makes it WAY easier to get them on and stay on.
I used and exacto knife and just put a small slit to let the air out.
Greased them and they snapped right on there and stayed.