Maybe this'll be clearer. The difference between a clit and a penis is minimal, aside from size. That's pretty much it, when regarding those two organs. Size goes by genetics and hormones in and out o'the the womb and how good your body is at, well, working. And between you, me and the wall, once someone reaches adulthood erect size depends quite a bit on hormones and psychology. One of my partners was an average size till she started hormones, in which case, now when she's aroused she's my size.
The difference in size between a trans woman with a penis on hormones and, say, you, is that she probably has more estrogen and less testosterone so she won't get completely hard. So it'll look smaller, though it won't actually be smaller since it still has the capability to get fully erect when given incentive through horniness, more T or both. Hence trans women juggling their hormones.
Here's what you can't do - you can't ask "What if women had penises" and then write off trans women because you don't like the answer. And why do I have the sneaking suspicion that most men here don't know the average size of a clitoris for an actual comparison?
(hint - there isn't an average size, really - runs the gamut) If you have a cis woman with a large clit, say, 2 or 3 inches and the clock turned back, tickety tockity tick and you magically switched it with a penis, (which is the same goddamn thing, so I gotta wonder if you know much about biology at this point) assuming she was healthy hormone-wise et cetera and so forth, her dick would probably be bigger than yours, since she had a large clit in the first place.
There seems to be an air of "if they did have dicks, they'd certainly be less grand than mine' here at the moment.
-and yes, I know quite a few women with large clits. They're about as rare as a thick 8 incher when you actually look around.