It’s big because it’s fighting against a lot of force and recoil from the firepower of the gun.

Generally, when you fire a high caliber gun, the force will push the gun back (into the person firing) and upward. Likewise, if you are firing a rapid fire rifle, the repeated firing will force the gun upward.

So, that foregrip is meant to be held to increase accuracy and limit the recoil. In the real world, there are other attachments to connect to the grip to help stabilize the gun.