I work at a small school and my kids attend it. Largest year group is 13. Largest class size is 23.
While children may be limited in their year group for friends, there is much more friendship between age groups than I remember from my own (large) primary school. I was one of 60 in my year - all my friends were in my year group and I don't really remember having much to do with other years. In the school I work at, there's none of that division by age. They might not have 60 friends of exactly their age but they have a decent number of friends, and because they are in mixed age group classes, and classes often do things together, they spend lots of time with these friends even if one half of a 'best friend pair' is in Year 4 and the other in Year 5.
I guess it's the 'family feel' thing that a PP kind of dismissed - what that means to me in practice is that (for example) all the Year 6 kids know all the reception kids by name. Lunchtimes might see a Year 6 child sitting next to a Year 1 and helping them cut their food. The children can sit wherever they want for lunch, we don't ask the older ones to 'buddy up' with younger ones, they just do it and you get a mix of ages all on one table, chatting over their meal. Which is much more representative of real life than the 'I am in Year 5, you are in Year 4, therefore we cannot really have much to do with each other' which I what I experienced at school.
When I moved to secondary, loads of my year group came with me. I ended up in a class with none of them. I knew only one other person in my class, from an out of school activity. So in the end, going to a big primary didn't mean I went up to secondary with loads of friends.
As someone mentioned, there is an issue with sports - we do football and cricket clubs but we don't have a school team playing sports fixtures against other schools. If your child loves that, then the small school may not be the best fit. We don't have an orchestra. We do however have a great music teacher who comes in and does keyboards and choir, and we have a lot of sports like dance and judo - all of which my kids love.
Ultimately the question is not 'small school vs big school' but which school is the right fit for your child?