In the book I was looking at, there wasn't a cover image. In Edit Book, it only has the title_page.xhtml, file0001.xhtml. There's a cover.jpg too.
The picture of the epub from my Kobo is here:
https://i.imgur.com/ZSSJR7a.png, which is from when I made the font-size: 2vmax. There's a margin on the left, but none on the right.
A workaround I found when testing is actually to add a cover image, which fixes this entirely! It makes the title page have the same margins as the story. The code I posted earlier doesn't seem to affect the cover image or the title page at all.