All drum sizes are not equal
Before looking into different drum capacities, an important point is that you can’t necessarily assume that they are all the same. If you take a dozen washing machines with the same drum capacity you would think you could fit in exactly the same amount of laundry in them all. However, this isn’t necessarily the case. One manufacturer’s 7Kg drum may have a smaller or larger capacity than another’s. My understanding is that they don’t all use the exact same criteria for deciding drum sizes in washing machines.