I've been working out quite awhile and this has always plagued me, skinny biceps. I'm not a small guy(6'2 185lb) and I can curl a decent amount of weight (45's) and I have large triceps, but no matter what I do my biceps stay skinny. They may enlarge but that goes vertical rather than horizontal growth. Is this a predisposition to my body type (tall and slim)? How can I get them to grow outwards, to become "fatter"?
Unfortunately, you can't change the shape of your muscles (you're not going to be able to morph your peak to width ratio), but you can work on the illusion.
If you're not already, consider adding reverse curls and hammer curls to your training program.
Both of these lifts train the brachialis and the brachioradialis (as well as the biceps brachii). Growing these commonly neglected muscles, and continuing to train the lateral, medial and long head of the triceps, will grow your arms' overall cirumference, and likely add the perception of increased "fatness".