Doing something similar now (gaining weight with the hope of putting on more muscle). Always have had trouble adding mass to my legs. But I figure that even if mostly gain fat, it's OK as long as I get stronger, which I am.

Something that has worked well for me so far has been keeping the reps relatively high (10-12). Previously I was told that lower reps (6ish) was best for developing more muscle and power, but didn't hold true for me.