I've been having this issue, or one just like it, and think I stumbled across a fix yesterday. I went from older Cinema Displays to new Studio Displays and it started. Screenshots were displaying at 4x their expected size in Quick Look, Preview, Textedit, Mac Mail... but displayed correctly in Photoshop. My best theory was they were capturing using the Studio Displays 5k resolution, then failing to compensate for that when displaying using Default Studio Display settings where the pixels are (?) doubled up.
After a tech support call and supervised screen sharing session, we figured out that creating a brand new user fixed the issue, Screenshots displayed at the expected sizes. But that's not a good solve.
We then did a screen recording of the issue, submitted a ticket to engineers, then got word a week later that this was "expected behavior" (it can't be) and radio silence for six weeks when my tech support rep asked for follow ups. I never heard back from him either.
This week I gave up and decided to reinstall the OS to see if that would work (it did not) and then create a new User from scratch and to migrate everything over. I spent about 3 or 4 hours setting everything back up again. Screenshots were working normally and as expected.
Then I went to set up Screenshot settings how I like them (jpegs, directly to the desktop) and when I turned off "Show Floating Thumbnail" so they'd save immediately to my desktop, the issue returned and Screenshots were now oversized again. Disabling that setting fixed them immediately. I turned this setting on and off a few times to be sure, then logged into my original User and re-enabled "Show Floating Thumbnail" and the issue was fixed even in my original user.
I don't know if everyone has the same issue or if this will work for others, but since I read through this for a solution a few months ago, I figured I'd add my update to the mix. If this works, it seems like maybe something broke with that setting in one of the recent updates? I'd like Screenshots to save immediately, but find them unusable for what I need when they display at absurd sizes so the slight delay is better than nothing.
If anyone else has success with this, I'd love to know.
