Having 32 cruises under my belt, my advice would be that star ratings are dependent on the facilities aboard rather than the quality of service. Spirit of Adventure is a small ship, no" golf driving ranges" or "rock climbing walls", just a nice relaxing cruise experience with good service, on a more traditional classic ship, cruising to interesting destinations and if the cost seems high it will be worth it. On the other hand if you want all the razamataz choose a ship with a high star rating and be one of the mass market. Happy cruisin.
It is due to the size of ship. Small ships have less facilities and less passengers to pay for the voyage. Hence companies like Fred Olsen and Saga have less revenue for a given itinery as their ships are small and Spirit of Adventure is at their small end. Hence the higher cost but should be well worth it. Been on 15+ cruises of all sizes (from Fred Olsen Black Prince to Celebrity Constellation) and both have their good points.
The review by Coulter earlier today sums it up well. Decide on your preferences, broadly smaller ships cost more to run than bigger ones.
I would add to Coulters reply - insurance and excursions are included in the price. You usually have to pay for these on top of the cruise price.