Look at your baby, not the scale. And, by the way, "slender" is not a bad word. A slender baby is a healthy baby.
My DD was 8 lbs. 7.5 oz at birth. Dropped to 7lbs.15oz before we left the hospital. It took her a full month to regain up to her birthweight, and she hovered at the 3rd percentile until she was 2, when she made a giant leap to 6th
At 5 years old she might weigh 35 lbs if that much. She is built exactly like her dad, tall and lean. Pictures of my DH when he was 10 or 12, you would say "how can those stick legs hold him up?"
So, although the ped initially pestered me about DD's weight gain, he backed off when he realized that supplementing and schedule feeding her made no impact on her weight gain, and she was always happy, healthy, hydrated and meeting her milestones on time, or early.
And, FWIW, I don't think she reached 15 lbs. until she was close to a year old!
Please relax and enjoy your healthy baby