I would say the answer would be the increase in the amount of matter present in the object. On earth there is a fairly good correlation between mass and weight. For example, a box containing diamond drill core will weigh more when the granitic rock is fresh and unaltered than when it is altered to lighter minerals like clay minerals (argillic minerals) as they are generally lighter than the parent feldspars like plagioclase, so the fresh granitic rock has more matter in it.
