I know that the length of the muscle belly limits the possbile size of the muscle. Do you have shorts biceps, with a long tendon attachment at the elbow? My wrists are just a bit over 7 inches, but my arms are 16 inches and lean. I have a long biceps however, only a very short tendon tying the biceps to the forearms.
Small wrists could mean a ectomorphic body which also has a rather light bone structure with short muscles and long tendons. But this is only a general statement.