"Max" is the new "Plus", it designates the larger phone in the lineup. And the X/XS is about the same screen size as the old Plus (slightly larger at 5.8 v 5.5) so the entire lineup was bumped up a screen size.
I expect the yet unannounced 3rd phone in the lineup (apparently "Xr") will be the same screen size as the old regular, in a smaller package, and a replacement of sorts for the SE.
edit: well I was wrong, apparently it sits between the XS and the Max, with completely different capabilities…
