A small class size can be nice.......but I always add the caveat that you don't want a small year group.
Lots of Preps have one small class per year (or possibly 2) and this creates a year group which can have very few boys or girls if it is a mixed school as there is rarely a 50-50 split. If you're talking very small, there is a small pool to make friendships with, to create competitive sports teams from and to create decent sized groups of similar ability for learning. Remember too, that the cosy atmosphere which might seem nice for a 4 year old might not be what you want for an 11 or 13 year old at the top of Prep.
Personally, I'd always want a year group of at least 40 children and 60 is better. As the children get older they will be taught by subject specialists and in sets for some subjects - so the size of form group doesn't necessarily reflect the size of teaching group. For example, a 3 Form Prep of about 60 per year may well run 4 maths groups or even 5.
People are often hugely attracted to small or tiny classes,mespeciallybwhen thinking of their 3 year old who might be in a tiny nursery group, but don't realise that actually a critical mass of children is needed for good group learning. The thing that actually makes the difference to learning is the quality of teaching not class size - this has been shown in research again and again, but people struggle to believe it. Yes, a smaller class should be easier to manage but a good teacher can manage a state school size class and ensure great learning for the range of abilities. The problem is that not all teachers are great teachers, in any sector. And actually you'll often find in small little Preps that there are some teachers who couldn't hack it in the state sector.
The trouble is, it is SO hard to know about the quality of teaching in Preps. They don't tend to do SATs and entrance to senior schools is often distorted by lots of parents paying for outside tutoring, so what evidence can parents look at it reality? I'd look at the marking of books and if they are setting individualised targets for children - you'd be surprised how many are still just writing 'good' on the work and that is it.
In the end, the thing I think is genuinely different is that there are fewer unsupportive parents who aren't interested in education and fewer disruptive children. People will pay a lot for those things and those things can mean better progress, never mind teachers, facilities or small class sizes. Most people don't want to say that is the reason though for going independent and often aren't really aware of it consciously as the key reason themselves.