Maybe I'm just tired, but was having a hard time following your post. Maybe bullet points? Hehe
I'm suspecting this is simply because the camera isn't pulling back to take the snapshots.
- The snapshot camera pulls back to fit the bounding box of the model.
- A bounding box doesn't exist without a collider on the prefab, so make sure there is some sort of collider added.
- The button that creates prefabs from raw models adds a mesh collider to each one, so those would meet the requirements.
Let me know if manually adding the mesh colliders works for you. I can imagine that there will be times where a user won't want/need a collider, so maybe I'll have the generate thumbnail class add colliders to any prefabs that are missing them, just for the photo. =)