That’s because ship classification is arbitrary. For Wet Navies the HMS Dreadnought set the standard for Battleships. Much silliness went on around the world war 1 era creating the need to find euphemism for battleships, since battleships were limited by treaty. Today, the need to have different classification for ships is largely wasted. Something about nuclear weapons making large ships effectively useless.
In Space however, Bigger is better, which leads to ship clasificaitons beyond battleships. I have No Idea how it started by at this point a Dreadnought is bigger then a battleship. (Probably something to do with how the original HMS Dreadnought just out classed everything when it showed up. Someone built a Really Big Battleship and It was the dreadnought all over again…)
Ship classes as I know them. Largely from David Weber’s universes:
Planetoid. Because sometimes It just needs to be the size of a moon… (And Dahak would eat the Death Star)