Big Fat Mutants are an evolution of the Muscled Mutant. With its very high health and high damage, it serves as a meatshield that can punch out just as much as it can take, in addition to being able to inflict bleeding or fractures.
The big fat mutant will move toward the player at a slow speed, lunging when it gets close like the Normal Mutant with a comparable distance. Its attacks are more deadly, and it has the chance to fracture bones or cause bleeding because of its attacks.
It has two distinct attacks. One is a simple straight hook using one of its fists; the other is a straightforward thrust with both of its fists. It will lunge a respectable distance (about the distance of a shotgun shot) during its punch, so being anywhere near melee range can hurt. Both attacks also have a distinct wind-up animation.
Due to its size, hitting it with an SMG is relatively easier compared to other mutants, so using an SMG can easily take it out while staying a safe distance away from its fists. Weakening it with a grenade before attacking it can help; then you can go in and finish it off with a melee weapon or gun. Also, don't let any areas infection get too high; otherwise there will be a lot of Big Fat Mutants instead of Normal Mutants.