We have. Dd was exlusively breast fed until 25 weeks, which was about 10 days short of the 6 months. She was 14 days late, which may or may not be significant! What we found was that whereas ds, who started solids at 4 months, was always difficult to get to eat, dd was really ready for food. She had started to get a bit grumpy, had stopped sleeping through etc. But she was SO happy to get solid food and eats very well. She's 7 months now and eats pureed fruit, veg, cauliflower cheese etc. yoghurt, baby cereal, baby rice, and has breast milk to drink.
The only thing I would say that has been difficult with her is that she will NOT take a bottle. So it is very hard to leave her with anyone. At nursery she has a very small amount of EBM (prob. only 2oz in a day though), so it might be worth getting your baby used to a bottle.
I don't think the size of the baby is a good indicator - it doesn't change their digestive system.
On the whole, I think the baby tells you when they're ready - dd certainly did.
Just saw your question - what were your reasons - well partly because of the recommendations, and partly, I have to confess, because of pig-headedness after ds never really got breast feeding sorted, and was on mixed feeding throughout, I wanted to prove I could do it, and also because the HV made the same sorts of comments to me. And I'm just a stubborn sort of person!