Hard grown can also mean that you intentionally give the plant the minimum amount of care necessary to survive and grow, but usually that is paired with harsh or extreme conditions so high heat, intense sun to bring out the plant's more rugged aspects. This does not necessarily make them bonsai that involves things like aggressive root pruning and what not, but hard grown plants do tend to fall towards to the smaller end of their normal size range. Especially in Agaves and Cacti it tends to make the plants look more fierce with better developed spines and teeth.
