FBR General Discussion, page-3145

The boom is carbon fibre composite to minimize weight so as to maximise block weight at reach. 22SBE is 22 standard brick equivalents or rought 4-5 cinder block equivalents.
A small excavator would have issues with reach once internal walls go up. It would have to build walls to full height before retreating.
They have HX110 built with the boom and are about to show us that it can lay bricks at length and height (4 storeys). This design beats a small excavator. Remember that is where they started from (15 tonne from memory)