"You don't look trans" and asking if you really are, or other reinforcements of how feminine you look.
These are compliments based on your physical characteristics that they find sexual because it is from potential partners on a dating site. It is very different than a platonic friend saying the exact same words because of the dating context.
It isn't indicating anything nefarious, just a sign that they are focused on that aspect and it is most likely a fetish for them. It often means they are just looking for sex and not a relationship. If they pivot to personality and other things about you shortly after then there is a better chance they see you as a whole person.