DD1 has probably grown more in the last 6-8 months than ever before! At Christmas last year I put her into her baby sister's clothes when she got muddy at the park and there was 5lb difference in their weights - there is 27 months between them and DD2 was a preemie too!
SO DD1 was 3.4yo and wore 12-18 month clothes!
She is 4 next month and is wearing 3-4 yo clothes and weighs 18kg - which I know as she needs a new car seat this weekend!
We have done nothing different - she has just suddenly shot up and is doing fantastically! Her consultant always used to say she reminded him to feel humble and to treat the child not the MRI scans!
DD2 is now 19 mo and is beginning to talk - she was only 5 weeks early and was a whopping 5lb 9oz (which, to us, was HUGE!!) She may not be a talker but in the 6 weeks since she has been walking, she has hardly stopped!
I do try to avoid milestone lists with them both - luckily, as DD1 WAS our first we simply has to accept that she would do things when she was ready - and she seems to do everything eventually!
They have both had us worried at times but they are both doing fabulously now and, even if I am biased, I don't think you would know (now) that they had such tough, early starts simply by looking at them and watching what they can do.
I hope your DD turns out to be so much joy and success as my two are - I count my blessings every day and love to watch them sleeping just to listen to them breathe (by themselves!!)