The babies size at birth doesn't have any bearing on lung development - which is why it's wise to wait as long as possible.

Lung development is one of the last things to be "finished off" - so what matters is when the baby would have been born if left to start to labour on it's own, which of course we can never know.

A 6lb baby born by elective at 38 weeks, who would have been born that week anyway may have no problems breathing - and a 9lb baby born by elective at 38 weeks who wouldn't have come until 42 weeks may have RDS.

Definitely best to leave them as long as possible. Every day counts at the end - not only for their lungs but but other systems as well.