I can actually answer this one. I’m a fraternal twin. I can confidently say that my dick is bigger than my twin brother’s. As for how I know that… As kids we had no issues bathing together… And later in life we just never cared about either being nude. Nothing sexual just a straight up lack of feelings one way or the other.

Also I lived with him and his gf for a bit and she accidentally walked in on me as I was getting out of the shower and her exact words were “I hooked up with the wrong twin”… Unfortunately for her I’m gay.