My youngest is also "big" for her age, but with delayed development - so her developmental age is a lot less than her actual age which is a lot less than her "size" age. She is 4 1/2 years younger than her brother but is only 2 shoe sizes smaller (she is 2.5, brother has just turned 7, she wears size 9 1/2 H shoe, brother wears size 12).
I find it very hard to deal with strangers who do not know her, do not know she has delayed development and judge her on her size as to what she should be doing - she only says minimal words, her co-ordination is not very good, etc. etc.
I am only 5'4", her dad is about 5'8" and she is 95 cm and weighs 18 kg (just under 40 lbs). Is in big size nappies - no where near toilet training - and it is very hard to find clothes that fit - she is not fat, just tall but built to go with her height.
She had a MRI brain scan (to check for any damage) but the hospital were also checking her pitituary gland to make sure she isn't secreting any hormones that are making her grow so fast - although she did sleep for 20 hours a day up to the age of 8 months - this means that she spent a lot of time growing and not much "developing".
They are also sending her for a bone scan as she is so big, and are going to do a lengthy development check in the summer so I will be given her (a) actual age, (b) development age and (c) bone age.
BUT at the end of the day I wouldn't change her for the world - she may be big, behind but she is so loving, sociable and lovable.