If you are using Mihon on Android, then you can change the reading mode for the series. Some series like to use small fonts, so I have them set to start each page zoomed in a bit. On my phone (and most probably), that means having it fit height instead of width since most phone screens will have a taller aspect ratio than the pages.

If you are on Mangadex on your phone…then, doing a little bit of experimenting, looks like you can change the scaling behavior by hitting the little bars icon in the upper right that shows up when you tap the middle of the page, then you can change it from fit both to fit height.

Also, if you are on iOS, then I don’t have much experience to help you out, but have been told that installing Mangadex as a PWA is the nicest way to use it.