A typical silo on a farm has many bands wrapped around its perimeter. Why is the spacing between successive bands smaller toward the bottom?

This has to do with pressure.  If you think about it, where is there going to be the most pressure on the walls of a silo?  It's going to be at the bottom.

The reason for this is that there is going to be all the silage at the top of the silo that will be pressing down on the silage at the bottom.  So the stuff at the bottom is really being pressured and it will push outward much more strongly than the silage farther up.

Because of this, you need more bands lower down to counteract the extra pressure.