I just returned from a 9 day cruise on the Seabourn Odyssey, one of the big sisters. The quality of the service is on a par with that on the smaller triplets with but a couple of exceptions.
The one that is most notable (though to me not terribly relevant) is that on the smaller ships the staff with probably know your name within two days, while on the larger ships the chances are most of the staff won't. Why? Because each staff member "only" has to learn about 100 names (200 guests) on the smaller ships, but about 225 names (450 guests) on the larger ships.
Otherwise, the cuisine is excellent as is the service. Seabourn has invested significantly in the Seabourn Academy to assure consistency among all of its ships.