Check thread 408-20283 for a full explanation of full versus reduced bore ball valves. Full bore valves are used when the pipeline must be cleaned by pigging, or when pressure drop is absolutely critical. There cannot be many applications where sufficient flow exists to "choke" a full open reduced port ball valve. The flow velocity would exceed normal piping design guidelines.
The Bore of the valve determines the overall Body size. In other words, it takes more metal to build a full Bore/Port valve than a reduced Bore/Port valve, thus more $$.
FYI Reduced bore valves can also be referred to as Standard bore/port.
Please notice that valves with reduced bore are cheaper against full bore valves due to their smaller torque values and consequently smaller actuators/gear boxes.
Be careful to offer reduced bore in case of pigging requirements for the referring pipeline.