I think it's the fact that trans women are more visible now, and people are actively "figuring out" if those they see are cis or trans.
My beautiful cis female friend is 6 feet tall, and she has gotten asked a number of times if she's trans. Honestly, she's about the most feminine woman I know, and could easily be a supermodel. Would she have had the same experience 15-20 years ago? Probably not.
For what it's worth, I was super excited that I was beginning to pass consistently as female, and attributed a great deal of it to my voice and height (5'4"), and then on the train a gaggle of guys kept pointing to my adam's apple and calling me an "it." There are countless features that could be pointed out as being masculine or feminine, and none are really definitive - there are, as we know, rare cis women who do have protruding adam's apples, just as there are cis women who are over 5'11".
Anyway, to me 5'9" is definitely not super tall by cis standards - I should get my cis friend to talk to you 😉 haha