Atom can't open files larger than 2MB IIRC, and files below 2MB are.. very rough. This was about 6 months ago, though, so hopefully it's improved since then!
EDIT: yep, things have improved: https://github.com/atom/atom/issues/307
EDIT: yep, things have improved: https://github.com/atom/atom/issues/307
But, yuck, sounds like I should keep using VS Code anyway. I kind of want to be able to use Atom, but 2 MB is a frighteningly low size limit.