Why is the MD-88's front starboard door smaller than its port one? Having been on a few airplanes of the MD-80 family, I noticed that the front starboard doors of the planes were smaller than the ones on the port side. Why may this be the case?

This may also be the case with the rest of the DC-9 and B717 family as well, but I don't remember that clearly anymore about those other planes.
The larger port side doors are for extra convienience during boarding. Passenger boarding is always done on the port side (left) while the starboard side doors are only used for servicing the galleys or as an emergency exit.