Mellifera races are very similar size.
How much they carry honey, it depends pastures, how fast they get their belly full. Then they must fly to home, how long distance. Next, how long the nectar stay in flowers and flowers arr full. If foragers clean the nectar from flowers in couple of hours, the rest of day they get nothing.
When you have 10 hives in apiary, there are allways "lazy" hives and "busy" hives, even if they are daughters from the same hive.
My best hives are allways big, and mostly they are hybrids between Italian queen x Carniolan drones.