Mega-acting is a term used in place of “over-acting” when meant as a compliment. “Over-acting” means you’ve gone too far, you have too large an amount of acting, like overfilling a glass and it spills all over the place and you have to clean it up. Mega-acting doesn’t spill because it’s a deliberate stylistic choice. It may be absurd, cartoonish, exaggerated, operatic, but not an accident. You didn’t overfill the glass. You purposely used a huge stein.
Classic examples of mega-acting include Nic Cage as Castor Troy in FACE/OFF, Nick Nolte in HULK, Nic Cage in THE WICKER MAN.