It really does depend on many factors
Even the owner's manual for the Miele 1930 washer clearly states that the maximum recommended load will vary with fabric type and soil load.
If you are washing perm press stuff, you do not want to fill the washer to max. Otherwise you'll get more wrinkling, which sort of defeats the whole perm press concept.
Heavily soiled laundry should also be washed at less than max load. It only makes sense. A smaller load will get more wash action and not overload the detergent component as readily.
It also depends on how well the washer handles heavy loads on spin.
Take my Neptune 7500, for example. When I wash large bulky items like a fiberfill comforter, I can stuff it in there and not have a wash performance or spin problem. But when I wash bath sheets (really big bath towels, I find that the optimum number is five. The washer will handle six or seven, but then it takes forever to balance the load to spin, and usually when it's over five towels I have to halt the spin rebalance process, open the washer, and do some untangling of the towels. Then it will usually spin to max extract OK. But I'd rather not have to deal with that for every load.
Smaller items also can be packed to max load, because they don't tangle as much and cause balance problems.
The Miele 1930 doesn't seem to have the balance problems that the Neptune can experience, probably because it's not a tilted tub design that tangles some items. I wash small white items (gym socks, underwwear, kitchen towels, etc) exclusively in the Miele, in part because they are generally smaller loads and also because it can boost the temp up to 170F, while the Neptune is limited to 130F. The Miele does an excellent job with these loads, and the 1600 rpm spin speed makes short work of later drying.
Anyway, I don't think there are hard and fast rules about what is the optimum load. It varies with the types of items being washed, and with the washer. Also, most all modern front loaders have adaptive water fill, so water/energy waste from washing a smaller load in a bigger washer is somewhat minimized.