I’m experiencing a similar issue in Mendix 8.12.1. It’s not really because of the size of the content of my popup, but more the size of my calling page. My page layout has scroll behaviour set to “Full Widget” as opposed to the default “Per Region”. Per Region causes too many scrollbars and looks odd. As soon as you do that and your calling page is quite long has a lot of fields, increasing the page height), the popup/confirmations appears at the bottom of the screen, sometimes completely off screen. I’ve logged a support ticket but thus far they’ve only said they can reproduce but have not found a solution yet.
Sounds like there something odd going on in your project. I've never had any issues with popups and just tested putting 25 fields in a 5.13.1 modeller application but no problems. The popup behaves just the way it's supposed to behave. Have you tried it in a more recent modeller version as well?
Must say that I'm actually more concerned about the fact that you've put that much info and data in a popup than the absence of a full-screen mode. Popups are meant to be used for a very limited amount of information usability wise. Just go with a regular page if there's too much to show. If you're sceptical about this then try scrolling a non-fixed popup on a tablet.