whoever designed the magic mouse hates hands for some reason, i don't think it could be made any less ergonomic than it is.
The Magic Mouse is low profiled because it is basically half mouse, half trackpad. The top half of the Magic Mouse is a multi-touch trackpad, which is better than a scroll ball since it never gets clogged up and has more functions for swiping & tapping.
The ergonomic logitech type mice are designed more so your hand rests ON the mouse. With a magic mouse you don't actually lay your hand flat on the mouse - you just put your index/middle finger tips on in a normal arch and hold the sides with thumb and ring finger while the lower palm area near your wrist rests on the mousepad. The low profile makes it easier to scroll/swipe your fingers around on top of the mouse without having to raise your hand.