Going back to this <b>non-canon</b> picture: http://www.furaffinity.net/view/12388376/
Just my take on how she might have become huge to begin with (at least temorarily), but I thought to myself that she's a rather grassy pony, and plants grow from water, and with this <i>special</i> water, she would grow from it. Ripping through her Sports Tape, Bandanna, etc. I think she seems to enjoy the power growth. She like getting more power anyway... It's kinda her style.
Earthsound won't react because she's a very calm pony. Even then, she knows Green Storm won't harm her own home. Nor would she harm Earthsound.