Well... I dunno Ashes. Same could be said of the Bretonian capital ship like. Sure, the gunboat and dessie look similar, but then the battleship looks rather completely different.
The other issue is that the gunboat was designed by Angel of Mercy then cruiser redesigned by Luis and now battleship made by Hades. Three different modellers mean three different models, but I'll be honest with you and say that I would actually hate having the cruiser and battleship being just bigger versions of the gunboat. It would be boring IMO. So as long as the textures are consistent with the rest of the current Hessian line, I think the models will look similar enough.
A lore excuse for this would be simple (if we would even need one): the Hessians take their engineering skills from a lot of different sources, such as: former Rheinland corporate staff, defecting Military scientists, sympathetic IMG miners, allies in arms in the form of the SCRA/Unioners/LWB/Bundschuh, and possibly even studying Corsair capital ship designs from their wrecks around the Omega systems. Something like this definitely wouldn't promote uniformity, but would promote adaptability.