Ok i bought new Pearl Izumis 2 pair... I bought mediums Im about 5'8"-5'9" tall and weigh about 149, I wear 32 waist jeans...I have a pretty skinny butt and I notice after getting off the bike there is some loose material righ above the pad in the back, if I pull the back waist band up its gone.
The guy at the bike store said i may need a small...I tried a pair on and I could get them on but they are REAL tight...I know they are supposed to be nice and snug but I think a small is too small...would a guy my size wear a small ??

a guy your size would probably normally be between small and medium.

where were the size small tight? if in the legs, you should try a different brand/model on. also note that higher-end shorts start to have bigger leg holes because... yep you guessed it... experienced/serious cyclists interested in buying premium shorts typically have bigger guads.

other solutions:
- tailoring the size medium. might be a little tricky, but I'm sure some tailors are confident in working with lycra.

- buying bib shorts. in those the critical fitting becomes strap length so that they fit your torso properly-- slightly loose might work, but too small is a huge issue.

you need bibs

non bib shorts will sag

they were tight all over, waist was pretty tight and the ......crotch area too...I really dont like bibd i tried them before, not my thing..

then you need to try some other ones. maybe a different size.

there's nobody alive that rides with bibs and then decides they want to go back to non-bibs.

I still wear both but never shorts on a long ride.

Actually I don't like bibs on a MTB either. Mainly because lycra can get trashed so easily in a crash on singletrack. Which happens more frequently than I would care to replace bibs.

a few things:

1. You don't want the small, you want a medium that fits better.
2. Try other brands, and buy some with more "panels." Cheaper shorts will only have like "6 panel construction" while nicer ones will have more. With more panels, they might contour to your skinny arse a little better.
3. You will change your mind about bibs soon enough....just as you probably already changed your mind about wearing those ghey shorts.

Is there another option for "non-bibs" besides shorts?

And yes, there are lots of people alive that don't like bibs. I'm not one of them, but you will hear from some shortly.

riding naked

When I got a pair, I was between size XL and XXL, the shop recommended the smaller size. They were tight at first, but as I rode more and more, they fit better and better. They weren't so tight as to cause problems, just a bit uncomfortable. I didn't notice them when riding, so I guess that means I suffering enough.

pretty close to me, i'm 31/32 in jeans, 140, 5'9" and skinny butted. i've tried both medium and small shorts, it depends on the manufacturer/model really. i can fit in both but i prefer the mediums, so i imagine you'd be the same esp based on the small being that tight everywhere. you could try bibs. i think my Louis Garneau shorts are a small but my PIs are medium.

not true. i rode with a bib and went back to shorts

Try another brand. American brands are made for people with large rear ends. Euro brands have a skinnier fit.