I don't believe Kobo does any resampling of the images the way Kindle will. It's always what the publisher provides, for better or worse.
I've got a copy of Satoshi Kon's Opus from Kobo (which has seemingly been delisted!), which was about 350mb in size and has a resolution per page of 1444x2072. Very high-quality.
By the same token, I have an
early epub version of
The Supreme and the resolution is garbage (546x816), but then, the upload date on that one was 2011ish.