Re: Fender BIG headstock "more resonance/sustain and tone"?
A bigger headstock puts a bit of mass there. It works on the same principle as the fatfinger, which means that it will affect some necks considerably, while on others it will hardly make a difference. Heavier tuners are going to do the same thing.
I believe this extra mass right there tends to lower the neck's own resonant frequency. Such has been my experience.
High mass bridges and sustain blocks do something similar, except they are mounted at the other end of the string, of course. The other difference is they conduct vibrations between the strings via the tremolo assembly to the guitar body and back. The headstock "just hangs there in the air" - so rather than a coupling point, it is an end point. Jackson fans are probably going to grin reading that, haha.
The smaller headstock on strats looks prettier to me.