I'm not the skinniest of dudes but I think I am only slightly overweight for my bodyshape and height.
I'm about 5'8 or 5'9, around 100kgs. Used to weigh more, and was slightly skeptical when my exercising hadn't
given me as much of a different as I had desired, but I have noticed my muscle tone change dramatically and
I am finding that I feel a lot healthier and stronger. My weight does still big me, but the more fat that is converted
to muscle, the better I find I feel, physically and psychologically.
I find myself passing a lot better/more often now and I'm not sure if that's my weight, voice, mannerism, etc. perhaps a mix of all? But when I was a bit heavier and not as far along on T, I had a horrible time passing. I think this may also be because when I was heavier, my chest seemed bigger. I have a pretty small chest and I can bind much easier now.
Everyone is different.

I think a lot of the trick of weight and passing can be tackled with finding great clothes that fit the way that will give you a more masculine appearing shape. I also think it's a trial and error process with clothes.