Lucky Magazine investigates this by observing 5 bras from different brands, measuring them at identically marked sizes and comparing results.
"To help figure out which size you are at what brand, we measured the end-to-end length of T-shirt bras from five popular lingerie lines—Calvin Klein's Level 3 Seductive Comfort Customized Lift, Victoria's Secret's Full Coverage, Gap's Favorite T-Shirt, Bali's Back To Beautiful and Elle MacPherson's Bare & Bold—to compare proportions. Consult the charts below to see how you size up. (Or down!)"
See the charts here: http://www.luckymag.com/blogs/luckyrightnow/2012/03/sizing-up-your-5-favorite-lingerie-brands#slide=1