Its not just you. After talcing the suit and practing donning multiple times, I'm able to easily slip right in wearing waterproof meshtech undergarments, which are a little bit thinner than other options. Just going up to 4e Arctics which are a little bit more lofty there is noticeably more work to get my right arm down and into the sleeve without stressing the zipper. I'm carrying enough BMI right now that dropping my caloric intake for a week or two would easily allow for thicker undergarments. I'm sure right now if I tried to stuff a 250gm or 400gm thinsulate suit in there with me I'd be in for some serious embarrassment. Thankfully, I'm finding (for me) that 40F dives for 45-50 minutes are perfectly comfortable with just the seaskin tech base layer, meshtec undergarments, and xerotherm glove liners. Pizza and beer are still on the menu this summer.