I have big BB's (like E-F cup at the moment) and they kept falling out the sides of my maternity singlets and maternity bra's, so I now wear a sports bra and it does the job of keeping the pads in place (prior I was always waking up with wet sheets and doona lol I leak so much the breast pads are always over full and splitting apart when I wake up in the morning).
Probably helps that I'm only expressing though as getting DD on latch on on a regular basis is a battle.