My older cup (the large one) sometimes leaked, more than just excess fluid from insertion/removal, but not enough to leak through my undies. However after a number of cycles using strictly the smaller cup, and am happy to report NO LEAKS! NONE! I leave the cup in for 10-12 hours on a typical day. Also, I started hormonal BC last month and the small Diva was a lifesaver for the first few weeks of spotting!
So perhaps if you are experiencing leakage with the large cup, go against logic and give the small cup a try! My guess is my vaginal muscles probably collapsed the larger cup soon after insertion, causing it to leak, but really, I have no idea why this happened!