Picky eater

Can some babies just not be big eaters? My 3 month old baby was low in weight at pediatrician visit. Not too much, but enough for me to get a lactation consultant. The consultant came twice and her evaluation was my daughter was improving and doing ok... Even if she eats sometimes for 4 minutes. She also hates eating in the morning and would rather play. She actually gained 9 oz in a week. Anyway, the pediatrician still was not that impressed but not worried either. I asked her if some babies just aren't big eaters, and she said yes. I feed on demand, yet the pediatrician said I should be feeding her every three hours. I'm confused as to who is right. Help 🙏

Ps baby has wet/soiled diapers frequently.