I think it's because larger babies are generally full, or nearly term, and are able to 'assist' their own delivery in some subtle way.. remember midwives saying this.. not sure how it works... whereas smaller, often pre-term babies, can't
Yeah I remember someone saying something about bigger babies being able to help in some way and that smaller babies were harder to deliver.
OK so maybe i'll go for 8lb this time but please no horrid stitches
A midwife friend of mine said the small ones get out of position more easily and have room to stick arms up etc whereas a bigger one just has to keep going.
Don't know how true that is but it does make some kind of sense.