Well that's what responsive CSS like bootstrap is for I'd guess. You can get that working in XPiNC.
As to the actual zooming. I don't think that's an Xpages thing at all. I'd think you'd need to use CSJS to tell the browser to "Zoom" in. I'm not sure if that's possible or not. XPiNC I believe uses Xulrunner browser which is the equilivant to an old FireFox. I forget which version they're up to in 9.0.1. They did surprisingly update it a while back.