Why are primary chilled water pumps smaller than secondary pumps?

PRIMARY CHILLED WATER PUMPS ARE SELECTED TO FULFILL THE REQUIREMENTS OF THE CHILLER WHERE AS SECONADRY CHILLED WATER PUMPS TO SUPPLY CHILLED WATER TO THE LOAD. FOR EXAMPLE TO THE FAN COIL UNITS & AIR HANDELING UNITS. IN NORMAL PRACTICE THE SECONDARY CHILLED WATER PUMPS ARE APPLIED WITH VARIABLE FREQUENCY (SPEED) DRIVE WHILE PRIMARY WITH CONSTANT SPEED, IN THIS ALSO PRIMARY CHILLED WATER PUMPS CAN BE USED WITH VFD SYSTEM. AIJAZ AHMED JALBANI MECHANICAL ESTIMATION ENGINEER EMIRATES TECHNICAL & THERMAL SYSTEMS L.L.C