You know the story behind that, right? President Kennedy had been to an Army Navy game (I would guess in 1962) and noted the disparity in size between the Corps of Cadets and the Brigade of Midshipmen (USNA had a larger authorized enrollment at the time). President Kennedy asked why and that got the ball rolling for USMA's enrollment to be increased to the same as USNA. Interesting for him to support the army, considering he was a Naval Officer in WWII.
I have no idea the answer to your question, but I think USMA is still authorized 4,400.
While class entering sizes during the 1970s and into the 1980s were often above 1,400 cadets, attrition rates as high as 40% prevented a class from graduating more than 1,000 until the 1990s.