can't stop comparing

Jennette • momma to a 2-year-old & a 6 month old
my little girl was born at 35+1, weighing 4lbs and 3oz. adjusted age, she's nearly 15 weeks old (3 months) and she's only 12+ lbs. i can't stop comparing her to my cousins baby, he's already 18+ lbs, although he was full term. i feel like i'm not doing what a mother should to help her grow. she's ebf and i would bawl my eyes out if i had to start her on formula.