Foundation 5 no longer using xlarge and xxlarge grid sizes

This has been changed in Foundation 6. In _settings.scss you now find the $breakpoint-classes list and add the extra classes you want. e.g.

$breakpoint-classes: (small medium large xlarge);

In your settings file uncomment and set the following as true (It's line 32 for me):

$include-xl-html-grid-classes: true;

Then recompile and the xlarge and xxlarge classes should be present. (Make sure to get rid of your overrides, just in case there's a conflict)