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.
Grains or other materials stored in a silo tend to spread out away from the silo wall and therefore exert an outward force on the wall of the silo. We can observe the effect of this tendency of loose material to spread out when we store them in heaps, which covers maximum circular area at the bottom and minimum area at the top of the heap.
The outward pressure exerted by loose material stored in a silo on the perimeter of silos is proportional to the height of material above any point in the silo. This height of column of material stored is maximum at the bottom of silo and reduces as we move up the silo. Accordingly the force exerted on perimeter of silo is maximum at the bottom and reduces as we move up the silo. Closer spacing of band wrapped around the silo perimeter near the silo bottom enable the silo to withstand this higher pressure on its perimeter at he bottom.