The California t-shirt conspiracy.
All of the t-shirts commonly available in California seem cut to make you feel bad about yourself. No matter your self-image, no matter your body shape or configuration, just about any t-shirt you find is going to make you feel fat. At the very least, most sizes run one size smaller (i.e., what is marked 'large' is actually cut as a 'medium', and so forth).
Upon reflection, this might be why personal exercise is so common in California.