They gain power, but not spin.
Actually, both...but setup alone can't turn their strokes to being really spinny if this is not result of the technique they use. If they hit flatter strokes, ball remains flatter regardless of setup they use.
This is why this has to be taught from the early age. In my view, using bigger head size is very logical approach. Playing with strings (type which allows better energy return) and right related tensions is another option to allow more power to setup. Now how to transfer that setup power to both spin and pace - it's merely a matter of technique.
I used to play with a 14x18 97'' 330 SW stock Prince racquet when co-poly strings arrived, before I took a hiatus. Poly string I used back then had a high energy return. The result was quite a powerful setup and pretty much all influenced it: powerful yet spinny string, 14x18 pattern which allows both lot of topspin and easy power even on moderate RHS and a decent SW. So, with that setup you had abundance of power, what you needed is to tame it...and you tamed it using a technique which transfers big deal of swing energy into spin.