The word narrow is normally used to describe width, but some people use to it in relation to depth. In terms of bathroom cabinets, both kinds are hard to find. Medicine chests are one of those things that tend to have a standard size. However there is sometimes no room to accommodate a full sized cabinet, width-wise, especially in smaller bathrooms. Looking online would probably be your best bet, as there tends to be more models to choose from than in stores.
I find this pretty cool – it actually goes inside the wall:
Narrow bathroom wall cabinet pictured: Broan-NuTone 625 Bel Aire Narrow Body Frameless Medicine Cabinet