He's getting the most surface area possible cooled this way. My cats lay like that when it's hot out. It's a sign that it is time to turn on the airconditioner!
Toeser lays like that too. He also lays in a really weird way where both his hind feet are under his butt and he's actually on his tippy toes with the bulk of the foot behind him. It makes his butt stick up really high and looks really strange.
Both my kitties also lay normally, but tuck everything under and make their haunches really defined so they look like turkeys laying in a pan. Also, both of them are the same color as a nicely roasted turkey so it's really obvious. When they do that we call them "Turkey Butt"!