I noticed when browsing images in manga mode on Pixiv, there is a new option on each image: オリジナルサイズ (original size) I think on old images this doesn't mean anything, as I found no difference double-checking some manga I came across. Seems with newer images (probably only after this option was implemented), though, it may actually turn up a larger image. I found out so comparing them on post #792979 and made sure to upload the larger image. The original sizes appear to have a URL like "_big_p0.jpg" rather than "_p0.jpg". The source URL's for these images also won't redirect properly, so I thought it was worth bringing up so that could be addressed as well.
Updated by animenutcase