You may need to put a piece of chain in the bucket like the draglines use to help release the mud suction and break it out of the bucket on the small bucket. May even work for the 16" one too.I have a 10" and 16" bucket. The 10" is no fun in clay soil when it is on the wet side. Sticks inside the bucket badly.
I tend to keep the 16" on most of the time...
With a welder and plasma cutter, have you considered fabricating a smaller bucket yourself?
Bill