Its the pet rule. Not matter the size if the animal, they will expand in the night to take all the coziness for themselves.

Reality is that the bed warms up while you sleep. Dogs and cats curl up to sleep to keep warm, so once the mattress is warmed up by your bodyheat, cat or dog doesn't need to curl up and instead sprawls out to find that equilibrium of warmth. So while the pet is probably very comfy to stretch out and take all the space, they might also be overheating some and trying to stay cool yet cozy. But they will absolutely try to shuffle you around every few hours if they can to take the warm spot you made.