It’s an accessibility feature!! I do accessible UI design for work, and I’ve always seen people complaining about how hard it is to click on certain UI buttons, or how they were easy to mis-click.
For example - because of the larger button size, the swipe-with-right-thumb-to-scroll motion no longer activates the swipe-for-quick-action feature of the share post button, meaning that people are less likely to accidentally DM each other posts while scrolling the dash.
And similarly, now that the username header is bigger, it’s easier to click the blank space around a username to get to the original version of a post/reblog. (I still think that nav should be in the kebob menu for REAL accessibility and consistent UX, especially since it IS in the kebob menu in the notes, but I digress)
As for the circle avatars, my guess would be that a project manager said: since you are updating the post viewing UX anyway, can we make the avatar icons more consistent with how they appear at the top of an individual blog?
Now I’m just hoping they apply this same accessibility update to the toolbars as well!