Hello Bronth,
The Invective Exotic Shotgun is intended to have a 4 round magazine. There is no perk that either increases or decreases this stat.
What you may be thinking of is the "Decreases carried Ammo capacity" text with the Send It perk. This decreases your reserve ammo capacity, but does not affect magazine size.
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Ok thanks! The shotgun had 6 rounds in the mag until I unlocked that node, hence the confusion.