the image file seems to be more or less the same between the two views, but it lives in a <div> which has a class title that gets a max-width:75% style property when the view is mobile.
I haven’t noticed this, but my use of the forum on mobile is more like an app. I don’t remember how I saved it on my home screen, but it’s probably via Chrome. I have noticed the app icon has progressively gotten smaller. The bird used to be too big for the circle, but lately:
I use the Discourse app on my iPhone (which I think is just a browser wrapper) and am seeing the squished logo whether or not I’m logged in, and I see it in Safari as well. Maybe Discourse hired the cinematographer who shot The Sandman? (don’t get me started) We haven’t changed the logo on our end. I’ll ask in Discourse Meta.