Thank you guys for all the responses, I'm happy that you are sharing with me your habits and opinions.
Well I agree with all you that having a big machine is convenient for some reasons. Especially when washing duvets or big things like this. I was just wondering why I have seen so many Americans underfill their front loaders, when you can fill it up without any problem.
Here is Slovenia we have similar habits as in Germany. I have a duvet covered with the duvet cover (that has the buttons). I change (and wash) regularly the cover together with the lower sheet and I put out of the window the duvet to air.
(I don't have any pets at home)
I don't use the duvet all the year around. I wash the duvet itself only once a year when the season changes.
We have the Frigidaire affinity AFT6000SE1 from 2006 and can put a ton in it, but many times I break up the load for drying, or some things I drip dry.