Who is saying only to tax the big grocery chains? It would also affect other industries (like oil and gas), according to the terms by which applicable price gouging is defined in the law. The point is to discourage unwarranted price increases by making them unprofitable after a certain point and according to specific metrics, not to punish specific industries. If you're not familiar with the concept, you could research it further and not just broadcast your assumptions or pretend it would be done in a vacuum where no other shorter term measures could be established.