Let's bring human female breast size into the discussion as well, another unique feature among mammals (not counting cows..?)

Here is just the first search hit[0] I found on the subject, linking the two.

"Finally, Morris (1967) believes that breasts mimic fleshy buttocks, thereby providing a clear face-to-face signal in upright bipeds, but he does not explain the evolution of buttocks in the first place."

I vaguely recall some assigned reading in junior high school science, which described how mounting from the rear (where buttocks were an attractive sexual signal to males) was transformed to sex from the front, with breasts becoming larger as a substitute for the buttock as a sex attractant.

[0]https://www.journals.uchicago.edu/doi/abs/10.1086/283699?jou...

Another: https://english.elpais.com/science-tech/2024-12-19/why-are-w...