I think it would be best if the druid's carrying capacity was fixed to the caster's strength stat, for both practical and fantasy reasons.
First and foremost, it's pretty annoying when you are carrying gear with no problems, and then shift into a low strength animal form, such as dire raven or cat, and all of a sudden can't move because you are now considered encumbered. It's almost like an added requirement to shift into those forms, not just to have the shapeshift ability available, but you also can't be carrying much on the character. It's needlessly tedious to require sending gear to another character just to use the functionality of the animal form, and then transferring the gear back when the usage is done.
And even from a fantasy perspective, I at least have never considered it as the cat actually physically carrying a backpack and all that gear, but rather that the druid's possessions meld into their form when they shift. So it can be justified that the new strength score no longer effects carrying capacity while shifted, because the gear is only being materially carried while in caster form.
I think it can be justified for gameplay sake that the Druid's carry capacity is determined by their caster form, but if you prefer to increase their carrying capacity while in higher strength forms (like "we made the bear actually carry the gear!"), I wouldn't be opposed to that.
