Putting the right amount of washing in the machines matters. Too much and the clothes just aren't going to get clean. If you are at all unsure please just ask, and the staff can advise you. The machine weights describe the wet weight, so it is not as simple as weighing the items before loading.
As a general guide, the machines can be filled to 3/4 full. If the items are particularly heavy such as towels, feather or jersey, allow more space as they absorb a lot of water.
We have two different washing basket sizes which you can use to give an indication. But if you are bringing in a full bag for life, you'll be fine using the 8kg machine, if you have a full Ikea bag, go for 12kg. A thin single duvet will fit in the 8kg machines, all double duvets should go in the 12kg. Full-length curtains can go in the 12kg machine but have to be washed one at a time.