ARGH, I knew I shouldn't have opened this flipping thread.
Weight has nothing to do with it, nor does 'instinct' (please use the correct word, 'instinct' would not tell us to go against nature).
DS1 is 7weeks old and almost 14lbs, he was 10lb 3oz at birth, this has nothing to do with gut maturity, whereas time does.
Babies develop at an average rate, not at all based on weight, but developmental age. It has been proven a gut is not mature enough until at the earliest 17 weeks, and at the latest 26 weeks, which is why it makes sense to wait until 26weeks, because you have no way of knowing which side of the scale your baby's internal organs develop at!
It has been suggested that if a baby can physically sit up unaided, pick up a piece of food, get that food into their mouth and swallow then they are likely ready to eat (Nature is clever, it wouldn't likely give us all the outer abilities to do something until the inner body was also ready), seems more than coincidence most babies will not be able to do the above until 6 months(ish) does it not?
Before six months they can't even chew, hence the need for purees. The only other animal that has their food mushed up is a bird, every other species doesn't eat food until it can chew, we are but animals.
As for having to introduce foods quickly Babies do not need food until about one year old, until then it is all about introduction to different tastes and textures, I don't see how six months to try new foods is going to mean it all has to be done at once.