Try looking at the tags link - it might give you a clue

I dont think they are more popular. Because, here, I made a mistake, I put "loced" instead of "locked".

"loced" word does not mean anything, there are no matches for it.

I've never really taken much notice but always assumed that it had something to do with the order that we put out keywords when we upload them.

(first post here brrrrrr)

To my knowlegde, in a tag cloud, the font and boldness of words are chosen according to their rank in the cloud. Some tags clouds even use colors. It works with a simple hierachy like :
16 not bold
15 bold
15 not bold
14 bold
etc...
so this means bold here just means ... nothing special. It's just combined size+boldness you want to pay attention to.  For instance, the first keyword is always on the biggest font (though not bold)

Hope this helps (and that I'm right)

Marc