I've also found that there's a possibility one may need the larger size regardless of childbirth... The 'keeper_cup' forum on Livejournal had several ladies saying women with well endowed husbands may need the larger size, because sometimes their pelvic floor muscles weren't as tight as others... just a thought.
I'm experiencing leakage, too, and am trying out the Diva size 1 because the Keeper gave me a latex allergy... I may switch, though, if I'm still leaking... for the reason stated above. =P