matt400,
The Equal product does not absorb water, this is true, but that's not the reason it's susceptable to moisture. It's the size of the powder granules. When moisture gets into the tire, and the tire cools, the condensing moisture will collect on any available object, in this case, the Equal powder. Each moisture coated granule of powder develops surface tension, and when it contacts an adjacent granule, a bond develops due to this surface tension. Due to the small particle size of these granules, a substantial bond develops that effectively binds the material together. Truckers that have this problem can be seen whacking on their tires with their jack handles to break the clumps loose, but this problem is aggravated in the winter, because this clump can freeze up.
This is why Equal distributors are so adamant and aware of removing all the existing air from the customers tires, sending it through a dryer, and reinstalling it back into the tire with the product. The ceramic beads, OTOH, are comparitively much, much larger, and circular in shape, and because of this are relatively unaffected by surface tension due to existing moisture in the tire.
The bigger the tire, the more chance you have of developing moisture simply due to the larger quantity of air in the tire that can hold moisture. I also hear (not confirmed) that IMI, the makers of Equal, will start pushing the application of nitrogen into tires using Equal, one obvious reason is to eliminate the possibility of moisture, the other to eliminate changes in air pressure due to temperature changes. But that's a different subject

Robert