I'm a 34C, and small fits me perfectly. It gives enough support for jogging, and it's also comfortable enough to wear all day. The wide band at the ribs and the Y-straps at the back keep it from moving around. It's a little on the expensive side, but I like this bra enough that I'm looking at buying a second one.
This bra is so great - very supportive and doesn't pull, pinch, or shift during activity! I have sloped shoulders, and the straps don't fall either, due to the Y configuration. Also, it has nice shaping so you don't get the unflattering look that many sports bras give.