From Diablo Wiki
The term "footprint" doesn't refer to a literal mark made on the ground by a footstep. In the context of Diablo 3, it refers to how large of a space a monster or character occupies, in terms of game calculations. Monsters that are physically larger have bigger foorprints, which means they can be hit by attacks over a wider area. Such targets are more vulnerable to AoE attacks, and may take additional damage from multi-shot attacks that spread out, such as Charged Bolt or Plague of Toads.