Tinstaafl,
You're correct if the volume of the tire changed considerably, and if you could measure the tire pressure when they were unloaded as well as when they are loaded. However, neither is true. What you do is multiply the pressure by the area that is supporting that pressure and you get the force due to the weight of the 747.
The reason that the volume change is considered negligible is because the pressure and volume can stay mostly the same, but the shape/size of the tire contact area changes. Of course, we don't really care much how the pressure changes because we only meter the laden tire.
The change in pressure is essentially what is happening with the oleos, but with the oleos, you do need to unload them first, so not as practical to do with everyday objects.