Yes about the combine tank and all other oat and corn holding tanks.
I was looking for an answer when I posted this.I really don't know the answer.
I have no answer except to go long with what one other says. Compaction. Oats must compact more than corn thus fitting more pounds into a smaller area. Corn must compact more than beans cause I can get more bushels of corn in the truck than I can beans.
I tried to find the true answer else where and had no luck.