Weird, the artist posted the same pic on Pixiv (https://www.pixiv.net/en/artworks/97119505 ), however, the pixiv version is WAY smaller in resolution than this one from Twitter(2088x1252 vs 1044vs626), I wonder why uploading a smaller version there? :X

The smaller size version is a 100 Quality jpeg and the larger size version is a 85 Quality version if you compare the two in https://duplicatebooru.herokuapp.com/

That explains the 270 kb file size in the large size and 363 kb in the smaller sized image.

Anyway it seems to be common to upload a larger image on twitter and a smaller one on Pixiv. The higher quality pixiv one is tagged as the "work illustration" and I'll guess the the larger twitter image is made from that one.

When Pixiv was the main platform for artists to share their work (5-10 years ago), the norm is to shrink them to 1000-2000px-ish size. Larger resolution is scarce. Since Twitter got popular, more and more are posting larger size there (which I honestly don't know why).

Maybe some still have the old habit (or even Photoshop action or whatever) to post them resized on Pixiv.

Well Twitter did up their bandwidth. Now as we know larger and higher quality images are allowed, but you have a point. Old habits die hard.