Is it normal for a 3 1/2 month old kitten to weigh 7-8 pounds?

I have 2 kittens from the same litter. I know the birthday so I knew what age to tell the vet when I took them at 3 months. The female was right at 3 pounds. The male was 5 1/2 pounds then so they scratched out 3 months on the chart and wrote 4 months. That was almost a month ago and the male has gained another 2 pounds! His frame is larger than his sister's and I can feel his ribs when I rub his sides. I just don't understand how 2 cats from the same litter could be so different. All cats from the litters I've raised were close in size regardless of gender.