I think the deal is that depending on the shape of your legs some dress lengths/styles are flattering & others make legs look larger than they really are. Even another inch or two can make a big difference in how flattering a dress/skirt is depending on the shape of the wearer’s legs. This is also true for women with very slender legs which can look like toothpicks in too short skirts.
I wish the Princesses had some guidance to help them with simple tricks like that to look the best they can given that they are frequently photographed.
That said, in Eugenie’s pix it’s clear she was running a private errand, & the angle of the photo is odd, so I think she looks fine in her casual errand running not expecting to be photographed dress.
As for Beatrice, I can’t judge her outfit’s length as it’s mostly covered by her coat.