I don't think that it has to do with breast size.
I have a U-top and I wear it with one of my less padded bras. I've performed in this several times and it stood in place without safety pins.
See:
http://www.bhuz.com/index.php?cmd=/p...ssoun/id_3858/
Probably the size of the U-top is just wrong. For me, it looks a bit too big for you.
MEISSOUN