>>3112182DoF is a fairly complex optical phenomenon that can't be easily expressed as a function of one parameter. Technically, DoF becomes smaller when zooming in, but only if distance to subject stays the same, which makes the comparison meaningless because the framing of your subject drastically changes.
Also, note that DoF is not the same as visual foreground/background separation. With the same subject framing and the same DoF (in meters), a telephoto lens will give more apparent subject isolation (pic related).
Also, different lenses having different kinds of out-of-focus rendering also affects how pleasing the focus separation will look.
