I stumbled upon this ability which doubles the Pokemon's Weight. What in the world could be it's use, other than receiving more damage from Grass Knot?
The moves Heat Crash and Heavy Slam are affected by the Pokemon's weight (relative to the opponent).
Aggron and Bronzong can both learn Heavy Slam, but not Heat Crash. Since they are already very heavy, this means the move will likely attack with 100 or 120 base power.
Aggron. If he uses the move Heavy Slam with this ability, it is deadly. Easy 120 damage on most Pokemon + STAB.