The lightest XGA projector is the 4.2 lb. Compaq MP-1600. The heaviest among the twenty smallest is the Sony VPL-PX1 at 8.8 lbs. Vendor marketing departments have understandably promoted the light weight of the projectors in this group as being vital for the mobile presentation market. Each new product breaking the 6 lb. barrier, or the 5 lb. barrier, gets a lot of attention because of the uniqueness of the achievement.
However, by the time you pack up a 6-pound projector with the notebook computer, cables, carrying case, hard-copies of the presentation, etc., you end up with a bundle that's a lot more than 6 pounds. So if the weight of the projector itself were to vary by a pound or so either way, it's not going to make much difference in the carrying weight of the whole outfit. Certainly, the lighter the better. But there are significant cost and performance factors in these products that are more important than whether the projector weighs in at five, six, or seven pounds.
Keep in mind that you may really enjoy the extra features and price/performance that you can get from a projector that is only a pound or two heavier than its competitors. So while weight is obviously a key factor in mobile projectors, we recommend not placing overdue emphasis on the exact weight of the projector.