Remember your DC will one day be a big 11 year old and what might seem great for a 5 year old doesn't always still suit later.
I've had a DC in a one form entry and then later in a 3 form entry. In my view, 3 form was better, certainly for junior age. As well as there being more children to mix with (and they did have English and Maths groups set across school, so they weren't always taught just with their class - so actually did mix with others) there were more children in each ability bracket, so that was good too. In small schools you might be one of only a couple of very bright or struggling children, which isn't always easy.
Larger means more teachers and usually more clubs as a result. It also means more money, which these days is pretty important when the funding crisis is biting deep - larger, with more children just probably gives schools a bit more flexibility about what to do with the cash and there will be some economies of scale to be had.
Larger means on the off chance that there is a major issue within a class, there is somewhere else to go (not likely to be needed, but it is a possibility).
In rural areas where schools are small, I know they can work brilliantly and am sure there are lots of real benefits to be had. Given a choice though, I'd go for the larger option.
Think again about the more laid back approach you've seen in the smaller school - might be lovely at 4, but is it what you want at 11....to be honest, they all have to up the anti pretty soon after reception anyway these days.