My original point was to show footing loads, but everyone so far has picked up on the differences in truss loads. Not to preach to the choir but all stick built roofs are installed after all the walls are built, including interior walls. Then the ceiling joist are placed, supported by the exterior & interior walls. Typically these houses are built with the main load bearing wall running down the center of the house, which sits directly over the main beam in the basement or crawlspace. This configuration will place 1/4 the total ceiling load on the outer walls and 1/2 the total ceiling load on the center support wall. (see diagram)
Now the the roof rafter loads on a stick built roof have multiple support configurations some are good and some are sloppy. If the rafters have no purlin supports or braces and only collar ties then all the rafter load will go to the outer walls. When you see vertical supports, purlins and diagonal bracing then some rafter loads are being transferred down which is being picked up by interior walls that transfers the load to the floor which transfers some of the load to the main support beam and so on.
The first picture shows a stick built roof with a hallway down the middle and braces termination on both hallway walls. The second photo is a large new house with purlins and braces scattered all over. (note: some of the purlins were under sized and bowing under the load) The third photo shows a purlin/brace system terminating on an interior wall, which could work if the floor joist under that interior wall is doubled or tripled depending upon the load. In most cases nobody though about how the loads would be transferred to the footing, i.e. the load path, and the ceiling and/or floors start to sag over time.
Back to my original post… The point I wanted to show is the footing loads on a typical house are not equal and can vary by a large amount. You can see over time the footings with the larger loads will tend to settle more than the others. If the walls and footings are adequately reinforced you will not likely see the actual settlement differences, but you may see some stress cracks develop in the walls. In theory you would vary the footing widths to provide a even pressure to the soil so the building settles evenly, but this is only done on commercial buildings that are custom engineered.
I hope this helps clarify the footing load issue…