For me, this fetish pushes two seperate buttons. First, it is a fun contrast to have a huge dick on a girly boy. Sort of like with futa, but a little more realistic, as in girly boys with big dicks actually exist out there. Second, and maybe this is what you feel Anon, is that there is unique appeal in the idea of having someone submit sexually who has a much bigger dick than you. Society constantly pushes this narrative that a having a big dick means that you should be sexually dominant. So, flipping that narrative has a certain thrill. I suppose it would be the same if it was a muscly buff guy with a big dick. But, since I am not into male bodies that much, a femboy or trap provides access to that desire to dominate a bigger dicked partner sexually while still finding their features attractive. Granted, there are people who want the dick dick trap or femboy to be on top, but that's not my taste, at least.