[Edi]

Regarding slings, they are nostrength for a reason: The power of the slingstone is not related to the actual physical strength of the slinger the same way the power of e.g. an axe blow is. The power is provided by the whirling motion and it requires more manual dexterity than strength to use the sling more effectively. A slingstone hurled by a man average strength will do more or less the same damage as a slingstone hurled by a very strong man simply because the mechanics of how the sling works and how fast you can whirl it before you run against the laws of physics render the strength difference fairly irrelevant.

That said, a Jotun would be able to sling a larger stone, which would cause greater damage at same range, all else being equal, so you get a bigger projectile impacting for the same speed, so when you figure Ek=1/2mv^2, a slingstone five times heavier will have 5 times the energy. It'll also have more momentum, so having a Jotun Sling with greater damage is justififed. Making slings strength dependent weapons is not, in my opinion anyway.