I'm in the same boat! DS has super chunky thighs.
I use PFs (he needs premium size, which is like the medium) and covers. We use Bummis super brights (M), and Thirsties- I can't remember what they call them, but the ones w adjustable rise and snaps. With the larger PF his bum is quite bulky so I've been looking for a few pocket dipes for going out.
I got a Fuzzy Bunz because they have the adjustable elastic. However, I don't like how the button on the inside that holds the elastic leaves a mark on his legs.
I am going to try a Tiny Tush next. I'll get it on Tuesday and write back w how it works!