If I visit any other Stack Exchange site from my phone, I see an easy-to-read version designed specifically for mobile. But when I visit boardgames.stackexchange.com from my phone, I see what looks like the normal desktop view; but because my phone screen is small; everything is tiny and it is hard to read and use.
I would guess that other sites simply have custom styling to support mobile; but they all seem to look the same as each other; so it seems like there is some sort of default mobile view provided by SE that Boardgames doesn’t have.
Examples of sites that look good on mobile: movies, philosophy, math, cooking, gaming... actually every site I’ve checked except Boardgames.