I flew with Ryanair in March and needed a seatbelt extender as it was an inch or two short. I've not had an issue before on other airlines and my size hasn't changed so I was a bit surprised but it wasn't an issue. On the return flight, I just asked for an extender as we boarded.
I flew with Ryanair last week, and again needed an extender but on the flight back the seatbelt fitted and I didn't need one - very strange!