If i had to make a guess, Weight = Mass x Gravity. So either you increase or decrease the Mass of the object or you increases its the objects attraction to earth (Gravitational pull), since increasing or decreasing either of those factors should directly result in a change of weight. Whichever seems more plausible, i'd say mass xDI am guessing since earth is the most dense form of the 5 elements once chakra would be denser and heavier when using earth jutsu because the chkara itself forms into that very earth. So by adding earth chakra into that solid object the density and weight of the earth chakra is added on to it which obviously increases weight.
And I am just going to take a blind shot and say that in opposition to the adding of earth chakra the person is removing earth chakra?![]()
Anyway have you read the instances these techniques were used? Like raising the impact damage from punches or stopping asteroids? xD