[ycombobreaker]

Funny that you mention corn specifically, because all contracts are not delivered the same even on-exchange! The deliverable grade of corn is "#2 Yellow at contract Price, #1 Yellow at a 1.5 cent/bushel premium #3 Yellow at a 1.5 cent/bushel discount" per http://www.cmegroup.com/trading/agricultural/grain-and-oilse... .

Along these lines, I heard there were complaints in the UK's LIFFE market because excess rainfall was damaging French wheat crops, but even the poor crops were within the exchange's loose contract specifications.

But I suppose this is a bit of a tangent. The takeaway is that everything is traded on a market somewhere, but there are different sizes and standards for markets. Private companies can have some number of institutional investors while still keeping reporting requirements low (IIRC Facebook ran into this limit before their IPO). Ergo, every company has a market cap, but companies with fewer investors/owners are harder to measure.