if your child's bmi percentile is in the healthy zone then congratulations! it sounds like your child is at maybe the ~90th percentile for height and a 50% weight so that might be around the 25th percentile for bmi. if the trend is good then your child's growth is tracking along a healthy path. children who have 2 parents with weight issues are at higher risk of obesity that is 13 times higher than the baseline so you are doing a great job. just like i would counsel parents with a child at the 75 or 80th percentiles not to worry likewise feel good about your child being at the 20-25th percentile.
my 2 year old son is tall for his age but only 50th percentile for weight. does that mean he is underweight?