It's quite simple really. You see, when they made TOS, they couldn't get Starfleet to loan them a Constitution class Starship for the filming. What with them being important ships and everything. So they borrowed a light cruiser from a different class that was roughly the same shape, painted NCC-1701 on it and used that instead, assuming people wouldn't know the difference, and couldn't tell that this ship was only 288m rather than 442m. When the show became popular lots of people assumed that the Constitution class looked like the smaller ship and it all got a bit confused until TMP was made with a bigger budget and they were able to use a proper (442m) refit Constitution for the visuals, then Disco used the proper, original design. The ship that had played the Enterprise in TOS became so famous as a result that it ended up in the fleet museum (under it's real name, USS New Jersey).
There, fixed it.