They are the same class of vehicles, they are big stompy things with guns, titans. The Malcador, Macharius and Baneblade are all super-heavy tanks. You could make a even larger tank, but it would still be the same class of vehicle. Their battlefield role is exactly the same.



Granted, Warhound-class titans are described as 'scout', but that is only relevant when there are larger, stronger titans about. Likewise, a Leman Russ could be used as an explorer for a Baneblade.



Now, Imperial Titans and Imperial Knights are different when it comes to fluff.



And yeah, Imperial Titans are larger than Imperial Knights...so far. The Acastus is close to the size of a Warhound, and it wasn't described as the largest Knight.