National Electrical Manufacturers Association (NEMA) is a trade association in the US who sets up technical standards. NEMA standardized on electric motor sizes based upon each specific horsepower, speed, and enclosure type to allow for easier replacement interchange between manufacturers. These size assignments are referred to as "Frame".
In the tables above, you will see the 3 different times that these standardized Frame sizes were revised to take advantage of motor technology and materials improvements which basically allowed for a reduction in physical sizes of the motors for the same horsepower, speed, and enclosure.
Keep in mind, that when replacing an older "U Frame" (1952 standard) or "Pre-U Frame" (Original standard) with the current "T Frame" size standard (1964 standard), we can provide you with Transition Bases which will place the smaller current frames into the same basic mounting position as the older frames. These Transition Bases are relatively inexpensive. However you will often have to replace your coupling or pulley bushing to account for the different shaft size.