I bought a new 8kg Fisher Paykel 6 months ago and it's fantastic, but before that I had a 12 year old one and that was 5kg and did the job just the same.
Mine has an auto water level so it senses how much is in there and puts in water accordingly....so I don't have to feel bad doing only a small load in my huge machine IYKWIM?
My mum has just bought the top of the line FP one and it doesn't have an aggitator in the centre. When I use hers I don't feel like the nappies are as clean....dunno if it's real or not - guess they get extra 'care' in her machine rather than getting bashed about in mine lol.