Or, it's possible Odo hasn't perfected his ability to form certain objects, so he might not get them quite right and they'll be lighter/heavier than they should be.
As a humanoid I imagine he has a somewhat solid surface, and underneath he's like a big jello mold and can adjust his density as he sees fit. He might vary day to day. If only Bashir saw fit to weigh him every day and compare the results :P
Liquid is HEAVY. (It sure changes my weight every month!) I can imagine Odo being heavier than he looks, since he kinda reminds me of a scarecrow with a solid torso and thin arms and legs. You'd think he's got nothing to him and bam, he might weigh like 260 lbs, all fluid.