Skunker, I did a quick read and I believe the small and large grids are only applicable to Foundation though, since I don't seem to see them anywhere else.
Here's what the small and large grids are for:
In responsive design, there are times where the layout of your website on the mobile version is extremely different from the layout of your website on the desktop because of the space constraints.
Foundation gives you the opportunity to OVERWRITE the "small" grid with the "large" grid for this functionality in case you wanted a vastly different layout, at a specific given breakpoint. Just resize your screen on the page that you have given and take a look at the first example.
If you put it into Code terms, I would believe the "large" grid has more specificity, and are just given a "display:none;" below a certain breakpoint.