The way Xpra works is that it's both a server and client in it's own right. So it needs to be aware for the final client involved to be able to learn about it from the other side of the chain, so it does need to be aware of it in order for applications to know that they need to adjust themselves. What this results in is that the client you're running through Xpra will just assume it's a normal 96 DPI display from yesteryear usually. I think you can do some stuff to tell Xpra that it should be a higher DPI but that'll also make it so that the application will be gigantic on a low dpi display, something that's pretty typical for X11 since it doesn't properly support mixed DPI environments (i.e. a laptop screen that's high dpi and an external monitor that is low dpi).