I was in 4 grade and the teacher was talking about how big and strong the Indian women had to be to carry those big jugs of water from the river up to the camp.  He said they must have had massiuve msucles!!  It started there!!

if we talk only about the "growth thing" and not the muscular part, i would say that it started.. in my dreams!

the first time i "saw" a woman become quickly muscular was in fact in one of my dream, when i was a kid... i didn't know very well what to think about it, i ve just discovered the world of muscular women...

then, i saw a fews pics that represented what i ve dreamed, like some she hulk sequence, or in some cartoons, like tiny toons...

I think maybe it was with some children's cartoons, mainly She-Hulk (naturally) in Incredible Hulk from 1996 one and the 1994 Spiderman series with the Black Cat. Maybe it was just he transformation 'fetish' first, but it moved onto FMG and .. other ones now. Shadowlurker's site and Manic's works helped too.
Or maybe I'm just weird.