My four year old is 30 pounds with a small waist and when he potty trained last year he was in a 3/4 diaper. I understand this problem well. I've used Gymboree underwear. They come in 2/3T. I found the cotton to be thicker and thus not stretch/bulk as much. Also, I had problems with Hanes, and other mass produced brands, around the waist. The elastic would come out and either loose all stretch or have elastic "threads" coming out rendering them useless. With as much as you have to wash the underwear, especially in the beginning, I wanted some that would withstand the wear. Gymboree can be a little more expensive, but you can find them on sale occasionally on line for $0.99 or the outlet/sales. Good luck with potty training.
Also, even though they are ridiculously expensive, Ralph Lauren jeans and most of their pants run smaller in the waist and leg and are longer. My son is a 2T waist and 3 or 4T length. I'm still trying to find more pants that will fit my skinny boys without making it look like they are wearing capris!