The DA page description claims 3840x2160 @ 2.79 MB
Your trick returns 3265x1837 @ 10.21 MB
So you get a slight downscale and a lot of hot air.
There is probably no way to solve this on your side, butt one should keep in mind that this is still subobtimal.
The other problem — the updated post claims 3265x1837
10.21 MB on the derpi side, but actually returns 1024x5760.7 MB. So the metadata was updated, but not the picture.
Got the fullres from the login screen. Good job DA
Huzzah, a new exploit (or at least new to me) to get full res pictures from DA! Thanks for mentioning.
1. Visit the page of the image you want while logged out (you can use incognito/private mode).
2. Click "Join" or "Log In" on the top. The image should appear behind "Join the largest art community…".
3. Right click the image and click "Inspect".
4. Dig through the code and find the image URL.
It's a pain in the ass (as is everything with DA nowadays), but for when you really, really want the full res, I guess it's an option now.
Sadly it does only seem to work on a few images, though. I tried it on around 10 and only found one other where it worked, the others only showed resized jpgs on the login screen. (Funnily enough, the one that did work actually sells the full size download for $25.)
Actually I think I've just figured it out
The side panel to the login screen, where the picture you came from is displayed, measures 416x690. If the budget res they're displaying, and therefore the res uploaded here, is lower than either of those two dimensions, DA fetches the full resolution to display instead (if it exists), since otherwise they'd have to stretch their thumbnail.
I just confirmed that all of the pics I've found so far were less than 690px tall, and I assume that if I was able to find any less than 416 wide, the same trick would apply.
This now raises the question: is there anything we can do clientside to make that login screen side panel larger, therefore increasing the likelihood we get served the full resolution?
Derpibooru costs over $25 a day to operate - help support us financially!