Some anatomists will tell you that more direct brachialis work will add some width to your arms. At least on the outside. If you want to try it do some incline
hammer curls as MRI studies show that that exercise engages the brachilais the most. But, there isn't much more to the upper arm than biceps, triceps and brachialis. Work them hard, while also working the major muscle groups like back and legs harder, and the gains will come. 2 months is nothing and 20 lbs over that period is really good.
Arm size is usually directly proportional to overall size so when you gain more bodyweight your arms will follow suit.