Weapon Size Question

[Saironthis]

If I have Gravity Bow <https://www.d20pfsrd.com/magic/all-spells/g/gravity-bow/> cast on me, and I use Divine Fighting Technique (Way of the Silent Shiv) <https://www.d20pfsrd.com/feats/combat-feats/divine-fighting-technique-c ombat/>

Do both of the size increases stack?

Or do they both count as a "virtual" size increase?

[Diego Rossi]

Way of the Silent Shiv wrote:

This benefit applies only to the first attack that you make against the target, and only with attacks made with light or one-handed weapons intended for creatures of your size (including thrown attacks).

Gravity bow wrote:

Gravity bow significantly increases the weight and density of arrows or bolts fired from your bow or crossbow

Arrows and bolts fired by a bow or crossbow aren't light or one-handed weapons, so you can't apply both bonuses simultaneously.

[Mysterious Stranger]

Also gravity bow is a personal spell and cannot be cast on someone else.

You could use a light or one handed weapon and cast lead blades on yourself. Since neither the spell, nor the feat actually increases the size of the weapon they would not stack. Both abilities say that the weapon is treated as one size larger. Both are working off the actual size of the weapon, so they do not stack.

[Azothath]

→Divine Fighting technique Norgorber Initial Benefit: Whenever you attack an opponent that is completely unaware of your presence (such as when you are using the Stealth skill or invisible), your attack deals damage as if you were one size category larger. This benefit applies only to the first attack that you make against the target, and only with attacks made with light or one-handed weapons intended for creatures of your size (including thrown attacks).
→Gravity Bow:T1(Personal) ... Any arrow fired from a bow or crossbow you are carrying when the spell is cast deals damage as if one size larger than it actually is. For instance, an arrow fired from a Medium longbow normally deals 1d8 points of damage, but it would instead deal 2d6 points of damage if fired from a gravity bow. Only you can benefit from this spell. If anyone else uses your bow to make an attack the arrows deal damage as normal for their size....

Divn Fgt Tech does not apply to many Ranged weapons which are generally 2 handed (bows, crossbows). The "as if" means the FAQ applies.

Gravity Bow operates off the size of the crossbow/bow. Again, the "as if" means the FAQ applies.

→CRB FAQ - Size changes & stacking, 2015 which says choose the largest.
The FAQ points out Enlarge Person works but that's for melee weapons.

[Pizza Lord]

Hand crossbow would probably work.

[Mysterious Stranger]

The weapon categories are light, one handed, two handed and ranged. Crossbows including hand crossbow are in the ranged category, not one handed. The feat does specify it works with thrown light- or one-handed weapons.

Hand crossbow

Cost 100 gp Weight 2 lbs.
Damage 1d3 (small), 1d4 (medium); Critical 19-20/x2; Range 30 ft.; Type P; Special —
Category Ranged; Proficiency Exotic
Weapon Groups Crossbows

Description

You can draw a hand crossbow back by hand. Loading a hand crossbow is a move action that provokes attacks of opportunity. A hand crossbow fires standard crossbow bolts, like those used by light and heavy crossbows.
You can shoot, but not load, a hand crossbow with one hand at no penalty.

You can shoot a hand crossbow with each hand, but you take a penalty on attack rolls as if attacking with two light weapons.