The head hitbox is rather large, and is placed roughly where the red box is. If your opponent is not twisting, or shut down, this will kill them very quickly. Though, you must be pretty much be no further than 11 or 1 o' clock from their front-- it is extremely difficult to hit at an angle because the hitbox is tall, but slim.
The arms of a marauder are easy to hit and worth shooting if they carry an LB20 or a gauss, or if they are already opened up, as they are excellent shields because of how easy they are to hit unintentionally.
It's better to aim further down to hit the legs, as the arms can and will get in the way of shots aimed higher up on the legs. Legging a fresh marauder is always advised, unless you are being targeted. The mobility is its other greatest strength, next to its hitboxes-- which we're learning to do something about.
The crotch plate between the legs does not move with the upper torso and is the best target for hitting center mass. If their legs are turned away, the green box above that is the next best place to shoot, other than the head. Shooting anywhere else aiming for CT, your damage will spread all over the torsos, unless they are potato and stand still. If you are elevated above a MAD IIC, the box at the top is the easiest to hit for CT.
Shooting within the yellow triangles for a side torso destruction is the most reliable-- the most surface area is there, and it is hard to torso twist and protect those fat torsos. Aiming elsewhere, you are likely to hit the arms and center torso if they twist. (Sorry that the triangles aren't exactly evenly sized, to anyone with OCD tendancies.

)
BE CAREFUL: the "shoulder" pads above the arms count as arm hitox. If the arm is destroyed, the shoulder pads remain, and will MASSIVELY reduce the damage you do if hit because of damage transfer. Avoid aiming at them.