As long as the resolution of the videos stays the same then the size of the PiP player is fixed. If you display a video with a different resolution then Firefox adjust the size to the new resolution (dimensions of the video),m so it sounds that this is happening in your case.

So how can we fix this? You may be right but there was no such situation before. It started to happen after getting update firefox but I'm not sure which version. It's a bug which should be fixed.