There are different types of motors, not just in size,
Size of the motor is based on the type of gun and Gear box,
You have a Short, Medium, and Long, and all three sizes come in either a Standard, High Speed or High Torque version.
Your standard motor depending on who the manufature is, will handle most springs up to the M110 level, but you will see a marked reduction in ROF and a reduction in the time that you can play on one battery.
High Speed motors, should be used in a high ROF gun, that´s the whole reason for installing this type of motor, they have a high RPM, but only a marginal increase in torque, best up to the M120 level depending on the manufacture.
High Torque motors are for High Power guns, M130 level and above, they will increase your ROF at lower levels of increased tuning, but the are designed to deliver solid performance when you are doing High Power tuning jobs, 1,5 Joule and upwards.
Remember if you are going to be doing the upgrade in motor, depending on what you are trying to achieve, a gear upgrade will most likly be needed also, don´t go dropping a high torque motor into a standard gun, your bevel gear will not be able to withstand the power for long.