Suit should fit snug. You cannot put clothing on under it except maybe a tee shirt, long sleeve tee shirt, underwear, maybe some cotton gym shorts or bike shorts, that's it. If you squeeze it, you might fit a fleece top, if it is not too bulky, to keep you warm, but more likely you need to just put a windbreaker or the top from a rain suit OVER the suit if you are too cold.
The critical thing is the armpits and the crotch. Should not be uncomfortable in either area. Unless you already sing soprano, you don't want the suit to turn you into one.
Also, fit in the shoulder area is checked just like when you buy an expensive (or not so expensive) suit or jacket (we're talking dress clothes here for you street urchins and bumpkins out there). Bring arms straight out from your chest, in front of you, palms facing down, bend elbows to 90 degrees, forearms crossing in front of you, and reach across your chest with both arms (like you are hugging yourself).
Does the suit let you do that without limitation? If not, the chest size is too small. If you are a small build this may not be an issue, but I have a big chest versus waist size and height, so I have to make sure the shoulder fit is not too small.
Hope that helps you out, sounds like the suit is not too big, anyway.