I have a design in mind for a custom trailer. I was on tour the fall of '20 and ran in to a fellow cycle traveler that was pulling a custom trailer. He made his with an external frame rail that allowed him to use conventional bike hubs where they are supported on each end (thru axle). That is one major design upgrade I am going to incorporate in to my new design. However, that is a larger project down the road.
Until then, one of the big challenges I have is rolling resistance. In order to overcome that I need higher pressure tires. to go to higher pressure tires I need to upgrade to 20" wheels. In order to get a 20" wheel set to fit the trailer frame I have now and be an improvement over what I am running, with the possibility of using them on the new trailer - I need to mount a conventional cone + race hub to the trailer frame.
no matter how I get there - the issue I run in to is there isn't an axle out there that I would trust in the application.
I am not concerned with the weight of the axle/hardware. That is a very small price to pay.
So to answer the question of making an over-size axle and needing one for weight - if I am making a custom axle for the application a few more ounces weight for an over-size axle won't matter so I might as well make a burly axle.
I could get a 20" rim and re-use the current hubs and axles. But they are cheap and I want something stronger that will hold up over some harder miles. Around town is one thing, but I have some tour ideas down the road I want to have something I can trust on a trip away from home also. If I break spokes or split a hub I can likely still get home here. If I'm 3-4 days out that is a tour end'er. Plus if I have some more conventional bike parts I can have a better chance at getting a replacement at a bike shop on a trip (another major reason for going to a 20" wheel set).