Aha -- I can help with this one.
I did a 6-month raw-vegan experiment last year and put on 10 pounds (which was definitely not my plan). I was eating a lot of green salads with olive oil dressing, avocados, nuts and nut butter, and especially drinking almond milk, which I made myself. Also, if you're vegan but not raw, don't forget tofu: the favorite food of sumo wrestlers! You could easily make a tofu version of chankonabe -- their staple.
This article has a recipe link:
Sumo wrestlers fatten up on chankonabe. - By Franz Lidz - Slate Magazine