12 inch thick pad here for all bins,  12 to 30 ton,.   As was stated, the minimum delivery gets most of it done.  If a larger bin goes in at a later date, you will always be good to go.  

Yes a couple of railroad ties will keep it from sinking in, but when empty, one strong wind ruins a bin, auger, and boot when it tips over.  

Cheap  insurance to pour a deep enough pad, that is wide enough to hold any size bin.  I assume that is the reasoning the manufacturer recommended the thick pad.