Why does white font look fatter on black background?

It always does, it's optical illustion. Some brands actually adjust the weight of their logomark to accomodate for that.

I believe this is an optical illusion. Take that same white font and place it over a lighter colored box. This should illustrate that the font is actually the same width.

Hi Kevin,

Does the white typeface have a white stroke? That would make it look “fatter” on a colored background.

Show your Appearance panel with your text selected if you don’t see it.

Jane

Could you post screenshots to illustrate the issue?

Is this a theoretical question?