Do you want to test your aunt more, or do something else?

Make her stronger

One fantasy you've always had is to be dominated by a muscular woman. Nothing turns you on more than watching a musclegirl flex, watching her skin ripple with barely contained power, the raw strength she has just waiting to make you submit...

Hardly thinking, you blurt out another bet. "I bet you couldn't even win a bodybuilding contest."

Her eyes narrow. "Is that a challenge?"

You watch in awe as her body begins to swell and grow. It starts at her feet, as you watch them tear her shoes to shreds as they grow ten sizes. Then her calves balloon outward, twice as thick as your thigh. Her own thighs follow suite, soon becoming larger than your head. Her hips widen, but seem small in comparison to the thick muscle that started to pile up on her top half. Your aunt's breasts all but disappeared, tiny in comparison to her dinner-plate-sized abs, her twelve pack chiselled enough that you could almost fit your hand into the divots between the square muscles. She rolled back her boulder-like shoulder, her biceps as big as two watermelons. Her footsteps shook the earth as her gargantuan 8' form towered over you.

"There," she snarled. "I'm bigger than any body builder in the world. I can win any competition easily."

You almost quiver in delight, dick at half-mast in your trousers. But there's something missing.