My guess is that your character is very small compared to the weapon.
First of all check that your character is correctly sized compared to a default cube of unit 1. Normally if you choose that a default cube is 1m, than you can make your character 1.6-2 Unity units (meters) tall.
If the character is correctly sized but the weapon is not then it's possible the Weapon model was imported with the wrong scale. Check the model asset and you should be able to set the scale.
Finally if you want even more control you can set the scale at runtime the moment the item is Equipped directly on the Item Prefab "Item Prespective" components:
