I am 250+, big in the hips and rear, DH is right around 400 and big in the belly. We have had no problems, I have found the the CM's are very sensitive to large people and adjust accordingly. We always get seated in the middle seats on boat rides (POC, Splash, Small World) and usually get that row to ourselves. The front car on RNR has a little more leg/body room, but even on a middle car we have had no problems. RNR has a test car in the chicken exit, if you have any doubts ask to give it a try- the hallway is very quiet and out of the way, you're not on display. The front seat on Space Mtn on the other hand is tighter than the other seats, DH had to switch with another passenger as the lap bar wouldn't engage. That was handled very well by the CM's.
One tip for rides with seat belts, grab the belt BEFORE you sit down, then as you are sitting bring it around your waist towards the latch- much easier than digging into your hipe for a belt as the vehicle is pulling away!
We have no prblem at all fitting together in any of the Omnimover rides- HM, Pooh, Peter Pan, Snow White, Buzz, etc. Simulator rides (Dinosaur, Star Tours, Body Wars) can be a little snug but not uncomfortable. TOT will be better I am sure now that each seat has it's own lap belt- the last time we were on it we had two little girls in our row, and the lap bar never made it down far enough to keep them in. They got lots of airtime and wanted to ride with us again!
Theater seats can be a bit of a tight squeeze, but I have found that Disney is consistently better than almost anyone with designing rides and attractions that EVERYONE can enjoy.