I recently got a new Microsoft Surface Go and experienced the "tiny" client problem for the first time, it is not Roat Pkz problem - it is a Java problem. I will make a topic for the fix since I tried to help people in the past, but it's hard without facing the issue yourself. It is nothing to do with changing your screen resolution btw, I'll edit this topic with a link when I made it.
Edit: just re-read your comment, interfaces are based on sprites, and I don't think they would "stretch" well in fullscreen mode for bigger screens, so them remaining fixed size and looking smaller on a big screen is sort of normal, but yeah post your examples