Is my baby too small?

At 28 weeks, my baby was 1.1kg. At 31 weeks 6 days (almost 32 weeks), my baby is 1.39kg (not even 1.4kg). My gynae says he is a healthy size because he is putting on weight, but i read online that ur baby should be 1.7kg at 32 weeks. Any idea if mine is too small? If so, how can i make my baby gain more weight?