I bought new Durano tires for my Cat (28 406) and bought the recommended size tube (20x 1 1/8) to go with them. The tubes are very difficult to use as they seem so much bigger than the rim. No matter how I try to put them on, they seem to crimp badly inside the tire at multiple points. I guess they are supposed to be like this but my original tire/tube combination seemed a lot closer to the same size and were much easier to work with. These are a lot like trying to stuff a live octopus into a jar.
Any thoughts?

Dianna is right. 406mm = 15.98 inches, while 451mm = 17.76 inches, so what you bought is an almost 2 inch bigger diameter. Most tires and tubes will also have the millimeter designation written on them, and that's the dimension you need to know when you buy.
 
The two different 26 inch sizes, 559mm and 571mm, are close enough that the tubes are pretty much interchangeable, but not so for the two different 20 inch tubes.