When the dumb Coneheads on Saturday Night Live talked about consuming “mass quantities” of food they didn’t know any better, but native Earth humans wanting an adjective rather than a noun should stick with “massive” unless they are trying to allude to SNL. “Mass” is often used by young people in expressions where “many” or even the informal “a lot of” would be more appropriate.

Expressions in which the noun “mass” can modify another noun, as in “mass migrations,” are fine; but when you can use “massive” instead you should do so.