Interestingly, in Star Trek Online "Dreadnoughts" actually don't refer to a specific class that are bigger than other ships. They tend to be specific subvariants of ships otherwise described as Heavy Cruisers (For Feddies), Battleships (for Klinks), or uhh, "Advanced Warbird"* that happen to be equipped with a "lance" type weapon. That is, a forward mounted BFG that can either one shot or heavily damage most any other ships.Yeah there does seem to be a bit of a brainbug where dreadnoughts in both scifi settings and by the fandom are just considered a class above 'battleships' in just being some super-battleship type of class. Which is fine with me of course. If your in space might as well go big.
STO actually does some very interesting things with ship classification spurred on by the more traditional Trek classes, and generally they make a lot of logical sense when you parse them down, even if some of the things are obviously done for gameplay reasons.
-----------
* The Romulans are kinda masters of understatement. Their "advanced warbirds" in STO are some of the LARGEST ships by volume that a player can use, and for some variants they're actually two in one ships that can separate into two more normal sized ships.