You're 5'7" I wouldn't say you're genetically wierd. Mine are about the same 13" flexed at 140lbs and 5'9" I'll bet your arms look bigger than mine with the 2" height difference. And with a 185 bench they're stronger than mine too so don't feel bad. My frame is so small that I actually look decent, like a regular athlete though not a bodybuilder. Needsize has it right it depends on the purportions. A better measurement would be armspan versus circumference since shorter arms would look bigger than longer arms of the same measurement.