It should indeed be the same carrying capacity as the druid herself/himself.
It's just silly otherwise, I've realised that I never shapeshift anymore even though I'm a moon druid...
Well, it doesn't help animal forms are underpowered garbage past lv2 without inheriting your higher AC and you even do more damage in humanoid with Shilellagh while having access to healing and ranged utility on top.
The sheer annoyance of being encumbered when you swap to anything but bear/wolf/deep rothe is just the cherry on top of the awful shapeshift design balancing.
