my baby is 7 months old and 7.5 kg.i am using pampers large size pant style diaper.my question is when her weight becomes 9 kg , which size diaper I should use?

A. yes, you can use diaper /pampers of any good quality brand. But not for the whole day. give diaper free time frequently in a day if possible use cotton langot, it will help to decrease diaper rash if any.  Always check every 2 to 3 hourly for urine and motion and change immediately once baby pass urine or motion. Size depands on weight of baby