I am under the impression that the mastodon sw comes with a .conf file where limit is preset to 500 but can be altered.
It has nothing to do with overloading the server as the smallest image is equal to 1000s of lines of text, not characters.
Some say it is don so messages display easy on a arm/android type of device, small vertical screen, but I assume it is little inconvenience to just rub the screen down some more to read.
Corps. are conditioning people to avoid reading