Creatures the size of rats or pixies are very difficult to hit, especially when moving. Assuming such a creature is active, attackers subtract 2 from any attack rolls directed at it.

Large creatures, at least the size of a rhino, are somewhat easier to hit. Attackers may add +2 to any attack rolls directed at Large targets. Truly Huge creatures, at least as big as a dragon, are +4 to be hit.

This ability is both cumulative and relative, meaning the inverse of the modifier applies to attacks made by the creature. So if an elephant strikes at a human, it subtracts 2 from its roll, but if attacking another elephant, the modifiers negate each other.

Savage Worlds Deluxe Edition p. 155