I agree with the rest of these posts.
Small classes or training with 1 to 5 people at my home dojo counted for 30 years of Go ju style where we would work on whatever each person wanted until that person was satisfied. Sometimes we worked sparring with - weapons only - for a few years. Always we had more sparring than in class.
I think it was priceless and very beneficial
BUT !
This is not good if it is your only type of training. Large classes with different types of people and instructors is totally necessasary.
I break mine down to 32 years of private workouts and 17 years during this whole time was in large classes.
This meant I worked out sometimes 4 to 5 days a week when doing both type of workouts at the same time. This worked out specially good for me because when I learned something new in class I would teach it to the guys - which helped me learn the new technique faster tnan the rest of the large classes.
I think variety is good.

I've never had one on one MA instruction but I have had private one on one fitness training with my sensi, expensive but worth it. Because it's a whole other game when your alone and you just can't cheat anyone but your self.