Hi itdepends,
You're on the right track with trying out a sample pack. One thing I would suggest is trying the sample pack from HDIS. It's $9.95 and you get 2 of every kind of diaper (fitted brief with tapes) they carry. They also have an underwear sampler if you prefer pull-ons. This is a great way to try out what fits your body the best, which does the best absorbing your accidents and which ones you like the feel of the best. Given that the mediums have not worked out for you, I'd go ahead and suggest trying the large size sample pack.
Another place you can get samples from is Tranquility who sells the ATN (All Through the Night) diaper. It's a great introductory diaper that tends to run a bit smaller in cut than other brands. In other words, their large is a bit smaller than other people's larges and may fit you a bit better.
Finding the right diaper for you can take a bit of trial and error but with samples, you can explore without outlaying a ton of money.
I'm partial to Abena, Molicare and Wellness briefs because they work so well and the fit is excellent on me. If you want quality, you can't go wrong with these....
my 2 cents worth,
CJ