Tag clouds are usually done by scaling font sizes according to some weight.

Actually this is not very precise. For a representative representation (lol, I should get this domain name. representative-representation.com), the tag size - that is the surface area! - should be a representation of the tags weight.

The suface however doesn’t directly depend on the font size, but is more like font size * length of word (length being appropriate for the font used).

So when displaying tags with very different font sizes, “egg” and “Technorati” shouldn’t just be scaled by their weight, but also by their word length.

OTOH, few users will actually be able to “grasp” the actual difference in size. IMHO it’s just about “popular” vs. “obscure” and about making the tool more intersting to use.