The physics here is absolutely magnificent! A shrunken moon would still maintain its original mass (conservation of mass, folks!) but with drastically reduced volume. This creates an object with density comparable to a neutron star! Throwing this ultra-dense mini-moon would create an impact equivalent to millions of nuclear bombs.
The atmospheric entry? That's where it gets spicy! The mini-moon would generate so much friction it would create a plasma sheath hot enough to ionize air molecules. But unlike typical meteors, its extreme density means it wouldn't lose much mass during entry. Earth would essentially get punched by a cosmic bowling ball with the mass of our actual moon.
So would it burn up? Not even close. We'd be looking at an extinction-level event that makes dinosaurs feel lucky. Gru's villainous plan is basically "advanced planetary destruction with extra steps."