But why is the Browse Activity tab now full of huge bolded text and gobs of whitespace? I wasn't unhappy with how it was, but this seems to strictly be worse, especially if you follow a decent number of people
Look at how information dense Excel can be with a typical spreadsheet, and the vast majority of users have no problem with it - yet when it comes to a basic page listing a few items, there's 50px of padding around each item in 12px font. Suggestions: remove the padding, group actions by the same person or same repo to remove repeated clutter, add descriptions of the repos in the front page so it doesnt require a click to see more, etc...
I'm not disagreeing with you, but is this perhaps the reason why this is happening? If we have so much more space, we've got room for extra big bars. If we get higher resolutions, smaller fonts are (possibly) harder to read, so we make them bigger.
Maybe I'm just making stuff up, though.