Girls - How heavy do you go for squats?

I'm using 15s right now, but thinking about moving up to 20s. I'm concerned that if I go much heavier, it will make my thighs bigger instead of leaner. I'm not sure there's any science to that thinking, though...

you're right there's no science to that thinking

being lean comes from losing fat. If you have a layer of fat over your muscles, you won't see the muscle, and fat is what makes people's legs look chunky. Lose the fat and you reveal lean muscle, which in women looks "toned" - the less bodyfat you have, the more you will see muscle definition. MOst women seem to like the look at around 18-20% body fat, where your body looks firm and "toned", but not the really heavy muscle definition that you see at 13-15% body fat (though of course many like that look too, it's down to personal preference and how low you want to take your body fat percentage)

using heavier weights will not make your muscles get significantly bigger, but it will make them a lot stronger and make your bones a lot stronger and make it easier to lose the fat, and also if your muscles are atrophied (wasted away), then at lower levels of body fat you will still look soft and half starved, not the firm, lean look that most women want.

Leigh Peele's blog has a free download on body fat percentage, so you can see what men and women look at different body fat percentages, and also the difference between someone with and without muscle at the same body fat percentage. Many women are afraid to use heavy weights because they think they'll look like a female bodybuilder (not that there's anything wrong with how they look - personal preference....) but you won't look like that unless you go down to 13% body fat... the same amount of muscle on a woman but a higher percentage (18-20%) women look more like bikini models without all that muscle definition but still with a good overall shape and looking firm and "toned" rather than defined.

So yes, I think you should use heavier weights... use the heaviest weight you can, if you can do more than 12 reps, it's too heavy. Or do a programme which involves lifting heavier and heavier weights, such as stronlifts 5x5 or starting strength. Lots of women have seen a huge amount of success from these programmes and you can find threads with lots of before and after pics, like this one: http://www.myfitnesspal.com/topics/show/977538-halp-heavy-lifting-made-me-supah-bulky