does half filling drum with washing ruin fabric over time?

Hello Andy. Several years ago manufacturers started to increase the size of drums and people bought into the idea that the bigger the drum the better. But unless you do a lot of big loads they can be uneconomical to use. They are only more economical if you wash twice as much as you used to at a time. It seems that many people just have the extra drum size in case there's something big they want to wash, but on a day-to-day basis they just wash the same loads they always used to. If you only quarter filled a 9 kg drum I would think that is seriously under loading. I would agree with you that it wouldn't really do the laundry much good.

Half filling a 9 kg drum is still under loading but 4 and a half kilograms of laundry is nearer to the average wash load for most people. So on the one hand it's under filling the drum but on the other hand it's not far off of a normal wash load. This shows why large capacity drums are often inappropriate for many people. If it isn't reasonably full, a lot of large capacity drums will not be able to balance the load properly. So they may refuse to spin some loads. This is not only inconvenient but can lead some people to believe they have a fault on their washing machine when it's just that there isn't enough laundry in to allow it to spin properly.