Its about angles. Two wheels going up at, say 25 degrees and the other flat is not pushing the car up 25 degrees. I will not bother to do Pythagoras but its less, so easier. Likewise once the front wheels hit the flat and the rear wheel hits the 25 degrees then it is, likewise, less effort. The push is of longer length, of course. The work done to raise the car is the same if all other factors are equal. No different to altering the gearing on a winch so its a comfortable pull for the operators strength. I am heavy and pretty strong so I can push harder than most people. So what works for me is not always going to work for them. However the above is the way to make loading a car easy and it depends on the wheelbases of the cars to load and the height of the trailer. Tricky as we are talking of Nobel and Isetta here, I think. Isetta are short and heavy so are always a problem. Add their unbalanced steering action and I used to hate the swine for loading in the van, as it was one of few cars I could not do on my own. Though I have had too on occasions. Nobs are long and generally well behaved, though they are a tubby little number unsuited to panel vans.
Trailers, for me the lower the better. Wheel diameter at least 10 inch, but as I choose not to speed with a trailer on, I do not need a huge wheel diameter to prevent failures, weaving or suspension issues. I now use a twin axle light weight racing car trailer, as I can sustain a blow out without endangering £50,000 worth of Microcar, Unlike a bad remould on a poorly welded caravan chassis - you've got to laugh at some of the £50 contraptions you see behind and carrying £100k of vehicle at events.
Ramps equal length so they are interchangeable or double up for awkwards like 4 wheel Isettas. Angle as least as possible for ease of mounting.