Muscle growth has to be triggered by increased stimulus. "ok for my size and weight" doesn't mean anything; if you aren't demanding more of your muscles they won't grow.
You also have to feed them. 99.99% of guys who hyper fixate on the size of their biceps (note as another poster mentioned, NOT the largest muscle in the upper arm) do not have impressive physiques at all.
To see any really appreciable size on your arms you will likely need to gain muscular weight all over while taxing your arms with heavy compound lifts as well as secondary isolation exercises.
Someone else asked how much you close grip bench. Spend some time working your triceps with heavy compound lifts like this and you will likely notice your arms looking much fuller and denser. Gain weight and lift heavy and they will grow. Otherwise, what do you want? There are no secrets.