As long as we are talking about enough weight to get past the stiffness of the tires, the footprint will be the same with either tire combination at the same pressure.  18.4's will not support it with 10 psi.  Neither will those old rotten 44's, but that's beside the point

I think the terra tires have about 15 in them because we don't want them flexing due to their condition (keep in mind the tires are only on there because we have them and they looked cool....there is no real application for them.)   15 will just about hold it up in an 18.4R38.  Assuming it will hold it up, the contact patch will be pretty much the same on either one, except for shape.  An 18.4R at 15 psi will have a contact patch that is very long, as they are pretty close to maximum deflection.  The 44's will only be about two lugs on the ground with that much air in it, but of course it is very wide.    Very wide times very short is about the same as not very wide times very long.