The Zoom sets are like little subsets of the "full" set for a given subject.
Imagine that a "full" set may give you everything right down to, in some cases, rivets and bolt heads. A "zoom" set for an aircraft kit may, for example, give you enough to dress it up a little, like instrument panels, seat harnesses and a few more bits and pieces, without going overboard.
The "zoom" sets, having less parts are often cheaper too. It beats buying a "full" set for a few details and leaving the rest unused.