One thing I can say about LINE Webtoon and keep in mind this is my personal opinion on them, it would definitely behoove them to have the same uploading guidelines as Tapastic. .Jpeg format is one thing, but asking that I upload 800x1280 sized versions of my existing pages for Life of an Aspie when all of them were made on a 940x1500 PNG sized format is another matter entirely. I tried to make my pages fit Webtoon's (imho) asinine uploading guidelines and they all turned out fugly when I uploaded them on Webtoon.
It's a general rule of thumb to make your comics large to begin with - it's a lot easier to shrink comics down than it is to make them bigger, after all. For Time Gate, I draw my pages on 11 x 16 inch canvases (which translates to about 4000 x 5500 pixels), and then when I save the JPG's, I shrink the size down to 940 and save the file (but I DON'T save over the original PSD file). This way, if I have to adjust my page sizes for Tapastic, LINE, or hell, even physical printed books (which also have their own "rules" to follow, even more so when you have to consider trim and bleed lines which you don't have to do when you upload online) then I can just shrink them down and not have to worry about increasing the size and compromising the quality of the original comic pages.
It might be of interest to LINE to offer the same page sizes, but they're not obligated to - they're their own website after all, with their own source code and layout and design, so they don't have to adjust their size dimensions to match those of another (especially COMPETITOR) website. By that logic, all comic websites should do 940 pixel width, including Tapastic, LINE, Comic Fury, SmackJeeves, DrunkDuck, Hiveworks . . . overall, it's definitely not "asinine" in the least bit for them to have their own uploading guidelines. That would be like going into Wendy's and calling their products "asinine" because they offer Frosty's instead of McFlurries
Besides, they're not asking you to change your page sizes. They're offering a free platform for you to upload your comics, and if it's not a platform that works for you, they're not going to actively "force" you to change your page sizes - you have to make that decision on your own
Think of all the people that had to change their page sizes to upload to Tapastic lol
That's just my 2 cents tho lol
Maybe its something I did wrong? idk?
If it helps at all, how are you actually shrinking up your pages? Are you making a smaller canvas and copying your image over and then shrinking it with the transform tool? Or are you actually changing the image/resolution size through the menu options to do so? (in Photoshop CS6, it's "Change Image Size", in Clip Studio Paint, it's "Change Image Resolution"). I change my JPG files from 940 to 750 very easily with very little image quality loss when I prepare my original Tapastic uploads for LINE. Your dpi settings might be too low as well (a lot of drawing programs default to 72 which is ACTUALLY ASININE LOL)
I would toootally use this method if I did vertical comics lol I know some vertical comics that do this when uploading to Tumblr - they upload each panel as its own high-res image :3