I too find that the games tend to turn out easier than usual. To the reasons already mentioned by RolandJ, I'd add:
1) Huge AI civs are rare.
2) More opportunities for tech-trading and selling.
3) More opportunities for stirring up trouble between AIs.
4) More opportunities for quick and easy wars of conquest.
Note, however, that points 2-4 are all
opportunities rather than inherent benefits to the player, and so make the game easier only if you have sufficient skill (and patience - checking the diplomatic options can get very tedious) to take advantage.
So for players who have trouble beating Noble (or lower), doubling the # of civs could well make the game much harder, particularly as you're much more likely to get boxed in early.
I'd also add that, on some maps, these games can produce nigh-on impossible starts where you're totally isolated on a pathetic scrap of land, while the AI civs trade through the tech-tree at a blistering pace.
Being 10+ techs behind by the time you meet the AI, whilst having only three cities and a very limited hammer supply =
