In my experience and as a rule of thumb, the larger the front lens element, the better depth separation (nothing to do with shallow focus depth). This is caused by the lens looking "behind" foreground objects, kinda the same way as a 3D camera setup... The light enters the front lens element from a bigger separated angle, making background, behind foreground objects, visible - causing better depth separation.
P.s. if above makes absolutely no sense, it's probably because of my poor english... Sorry...