I'm not sure exactly, but I can remember some distinctive moments from the fourth grade.

There was a girl named Alicia that sat behind me in class...  I remember that I was about average height and weight for a boy my age...  About 4'5, 75 pounds probably.  SHE, however, stood 5'6 and weighed 100 pounds.  Seriously, she looked like a very youthful, flat-chested adult.  Every day she'd pick up my chair in as I sat in it and tell me how small and weak I am.  I tried my best to ignore it and not to make a scene (because I knew she could wreck me, and I kind of liked the attention) but that moment stuck with me. 

When I began my first sexual fantasies in sixth grade they involved making a girl of my class, Katie, who was stacked at size C, tall, strong and bossy like Alicia.  Ever since that encounter most of my growth fantasies have involved characters that share the same attributes as Alicia did.

I saw Alicia again at the super market after graduating high school.  She sat at a bench outside.  Judging by her size, frame and musculature I'd say she's about 50 pounds heavier than me.  She's about a full head taller than me, last I knew, and her breasts were nearly as big as my head.  She hadn't graduated yet;  apparently she was held back a year because she was teasing some boy in junior high and he hit her, so she hospitalized him.

It helped with the fantasies that while I was growing up the girls in my high school and junior high were a little stronger and taller than average.

Yeah it's obv. hard to believe.  You'd have to be there.  But I live out in a rural community, and the farm girls get pretty big and strong.  If the average woman is 5'4, their average is probably 5'7-5'8.  The schools mix rural, "urban" (if you could call it that) and suburban lifestyles so the people are of diverse sizes.  Generally a farm girl raised on organic milk vegetables will grow to be taller than a guy raised on McDonalds and soda.  There are lots of sports programs for girls too.  The fat ones do cheerleading, the fit ones do lacrosse, volleyball, football, basketball, etc.  If the guys aren't on the football or wrestling team then it's a safe bet that they're inside all the time playing video games.

Too many cartoons and the scary monster that is the internet.

That's what started many growth fantasies.

Like Economic Growth Fantasies 

Interesting. I'm sure that has something to do with it.

For me, it started at a very young age, say no older than 9 years, possibly a little younger. I had my first "girlfriend" at that age. That was at a time when you never let your male friends know you liked a girl or suffer endless ribbing. She was cute and since I didn't have any sisters, I held that childish wonder of her being a girl and "different" than a "boy", if you know what I mean. She didn't "act" like the typical girl with me. She liked to do "boy" things like climb trees, run around, somersaults, wrestle, she even liked to do chin-ups from a tree branch. Not play with dolls or such, at least not around me. We were very close in height and weight. She was strong, for a girl, I thought. She even flexed her arm for me and asked me to feel it. And I think we did a little childhood exploring with kissing. Anyway, I liked her and I do remember feeling it was great having a girlfriend like her.

I think I carried that experience with me into my early teens. When I was discovering my developing muscles and wanted to start lifting weights. When I started feeling those first workout muscle pumps, and being a teenager, it was a bit sexual. I then understood  what Arnold S. meant with his statement of working out was better than sex. Although I didn't think it was better than sex, I thought it would make great foreplay. I remember my fantasies shifting from mostly amply breasted girls toward wondering if a girl were to exercise with weights, would she feel like I felt? Would she feel her swollen pumped up muscles? And would it turn her on? This was at a time before female bodybuilding at it is today. I think that was the start of my female muscle growth fantasies which have persisted to this day. From that time if I see a girl or woman with bare arms, my eyes are drawn to her arms in hopes of seeing some beautiful muscular lines.