consider that the body is full of lungs and airsacks and hollow bones (i think, definitely lungs at the very least) while the tail is basically just solid muscle, and iirc the pivot point of the hips is actually a pretty good deal further forward than you’d think.

They also don’t have to be completely evenly balanced, it’s normal for there to be some muscle effort involved to stand up, that’s why animals generally like laying down when they can.