In Rainbow Six: Siege, the hit boxes were tailored to each uniform skin. There was uproar when players discovered this.
The players demanded that each operator must use the same hit box design for all of the Operator's uniform skins. Devs then made changes to hit boxes accordingly. Which often made the hit boxes slightly smaller than the uniform skins, because the skins had different distinguishing features such as headsets which the hit boxes did not have.
This then led to players complaining that they were getting blood spatter from headshots but no hit registration. That was because the new hit boxes had been made slightly smaller, but the animation from the old larger hit box still played. Which was probably because the blood spatter animation was actually linked to the appearance of the Operator rather than the hit box, and the appearance of the Operator was still the same size as it was before the hit box changes.
@Huntinwabits So, yes, skins can have something to do with hit boxes.