A "small compartment" refers to a small space or area that is used to store or hold things. It is a limited and enclosed section where objects can be kept in an organized manner. Full definition
The vehicle has plenty of storage space and small compartments for the smaller things. (autolist.com)
The rear hatch opens to a large space with a compartment under the carpet with smaller compartments to store tools and emergency supplies. (autolist.com)
There are two smaller compartments on the outside of the bag. (businessinsider.com)