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.