Skinny biceps but big forearm?

I do a lot of water polo and swimming but for some reason, my biceps have stayed skinny and my forearm is big. i know that lifting weights gives you bigger muscles and all, but even after i lift weights, do push ups, etc, they still seem to be skinny. is there a certain workout that i can do to increase the girth or overall size of my bicep or something of the sort?