Anyone who played FFXI or XIV's dat system knows the answer to this.
The .Dat Files that are used to store data for the game encorporate huge chunks of graphics for the game into one singular semiencrypted file. a .dat file.
Each .dat file houses different chunks of graphics. One .dat could house all of the UI icons used for status effects. One could house all the animation skeletons and pathings. One could contain all of the textures used for the environment.
As such if even one small change is made to this file it must be updated. SE doesn't currently have a system that can unpack, update, repack these files on the fly on your desktop when you patch so you have to download the entire file even if only one small texture out of 100's of possible textures packed in is changed.
When they add a new status? The dat file that houses all the status icons is gonna be updated.
When they add a new class? the dat that houses all the class icons for the UI is going to be updated.