On average, she'll notice the difference but you probably won't. If you're thinking that by your having whichever girl it is you want to take the pill you'll transform a flat chested runt into a voluptuous porn-star lookalike with breasts the size of watermelons, you have another thought coming.
From experience, the weight fluctuated from 3-7 lbs, and didn't increase breast size significantly.
Regarding the comment about men taking testosterone pills and becoming huge, if it were only that easy...