I think it may have more to do with the VESA mount size more than anything. Some brackets can take a decent weight, but only have 400 x 400 VESA mounts (probably up to 60ish”). Some brackets then go up to 600 and 800 sizes which are found more on the 65”+ sizes. I’m just guessing though
Good point, but then the important thing would still be the reference to the max VESA size for the bracket, rather than the TV size, as not all TV sizes have consistent VESAs.
The other thing I thought was for the brackets that pull out and swivel the viewing angle, the size of the TV will reduce the max swivel angle. So manufacturers might reference a max screen size to prevent disappointment in the swivel ability.
But for simple fixed brackets I sure now that TV size is irrelevant (VESA issues aside), its the weight thats relevant.