Capacitance is a term used to describe how well a capacitor can store charge. The capacitance of a capacitor depends on several thing. because a capacitor is a two plates separated by a distance then both the area of the plate and the distance between the two plate both effect the capacitance of a capacitor.
Capacitors stores electrical charge on two plates, if the plates were expanded in area, then it would seem the there is more room charge, therefor increasing the ability store charge.
To understand how distance between the two plates can effect the capacitance of a capacitor, we need to examine how a capacitor works.
So if the distance between plates is small then the electric field get up to strength sooner, with less charge, "reducing" the storage capacity of the capacitor (and reducing the capacitance).