I noticed a long time ago that nuclear weapons’ yields were measured equivalent to a number of tons of TNT exploded. This is a reasonable yardstick, I see now - TNT can be made very pure and all of its latent energy is released in a quick, easy-to-measure bang. I have no idea how the yield of a nuclear weapon is gauged (many seismographs at known distances from the blast giving readings?), but a known amount of energy is released in a nuclear explosion.
Why are conventional bombs not measured in standard units like this? Perhaps they are, but when the information on the MOAB was released, there was no information on its actual capability. It’s a big thing, definitely - but I had a look at http://people.howstuffworks.com/moab.htm and they carefully explain that the MOAB’s explosion is much smaller, give some figures and facts about a nuke, and then give no info on the MOAB’s blast.
There’s a bit on the third page about the preicse mixture of the explosive in the MOAB.