From experience, female clothes aren’t proportioned to fit trans women as well as cis women. While in your other comment you make a good point about some cis women also being outside the ‘conventional’ physical expectations for women in western society, that doesn’t also mean that trans people don’t face the same issues. We talk about these problems from a trans perspective because trans people are often targeted with legislation and rules from people who don’t understand, and are blocked from being treated as their preferred gender. A bulky cis woman might share physical characteristics with a trans woman, but their existence is also significantly less opposed.

Edit: to my first point there are a number of biological size/proportion differences between cis men and cis women that can be seen here: https://ehs.oregonstate.edu/sites/ehs.oregonstate.edu/files/pdf/ergo/ergonomicsanddesignreferenceguidewhitepaper.pdf