The larger ships are more than just more passengers, they also have more space for public rooms and other passenger areas. Crystal has the included Nobu supervised restaurant as well as a sushi bar and equal quality Italian restaurant, a large internet cafe in addition to classroom full of computers, and possibly more rooms with different types of entertainment, afternoon tea, dancing, etc.
The value of a cruise is really a personal perception. I haven't seen bathrooms in standard cabins that are bigger than Regent's, for example, and Seabourn's included caviar service is to die for. The important thing to figure out is what makes a cruise a good value for you. If you enjoyed the size of the ship and passenger capacity on Regent and Seabourn, that may tell you that those ships are a better fit than Crystal would be.