These do run a bit small, I'm a size 10, C cup and the XL fits just right.  I can jog in this bra and it doesn't give you any "bra bulges", making it just right for T-shirts.  There's enough support to hold the girls in place properly without being binding like some sports bras, yet it's light and breathable.  It took a long time to find the perfect bra that suits all of my needs.