The thing is, Ranseurs and Sarissas are suppose to counter Legates and Murillos. Since the Legate is a huge pice of metal and the Murillo is sleek , I don't really see why the Sarissa - Ranseur couple couldn't be like that. Making the Sarissa more compact makes it I bit more agile I suppose, somewhere near it's counter battleship , and somewhere near the Mako , which it might meet in the Omicrons.
On the model, it actually looks better with being short , it is a nice difference between a dreadnought and a battleship. However, a retexturing and some fine tuning to match the OC snub line could help a lot.