...CurlyDave (& others), could you explain more about why the hydraulic thumb won't work with your 12" bucket? Is the thumb too wide? Thanks.
The thumb itself is not too wide, the thumb mount is too wide.
I have a QA backhoe bucket, and the QA assembly is pinned onto the dipperstick. The hydraulic thumb is mounted on a common pin with the QA assembly. (This allows changing the bucket, while the thumb stays attached.)
Part of the thumb install kit is a longer pin, which has room for the thumb mount. The pin is longer than 12", about 14" (from memory). If used with a 12" bucket, the ends of the pin would scrape the sides of a trench.
I don't know if Kubota thumbs are the same, but I suspect they are.
Given that the 12" bucket packs with clay and is a general PITA, I happily sent mine to Jenkinsph in exchange for fabrication he did for me to mount a hydraulic auger on my backhoe.
I have either a 16" or 18" bucket now, and it works very well.
As I see it, the only real disadvantage of the bigger bucket is that if I want to sand or gravel bed a pipe in a trench, I will need to buy more sand or gravel. To me, this is not a big deal.
Smaller hoes may need smaller buckets just because of digging force issues, but I have enough power and have never wished for the 12" bucket.