Either you send mega-goblins, or you send MORE goblins.
A lower level party might fight 3 goblins fair and square, so 4 levels later they confront 6 goblins and 2 lieutenants.
The idea that the same enemy stays a challenge despite the level increase is actually what I despise in D&D. My character has grown in power, why is the rat from the beginning still able to down me?