Definition has to do with being lean. A lean/(defined due to low bodyfat %) 16.5 arm is going to look bigger than soft 17.5 arm.
Regarding slow progress, usually people do too much for chest and arms because they are "show" muscles. Try backing off a few sets for a week or two and see what happens. If that does not work, change your split, if you work them once a week try twice or if twice than try only once.