Please keep in mind that for this particular metric 'consistent results
across all operators' are not necessarily to be expected. As RSSAC-002
itself also explains, the goal of providing this metric was "to detect
any trends in the growth of the zone". Of course serious issues like
truncated zone files are a different thing altogether, but size
differences related to compression differences could occur.

Personally, I would have preferred to keep track of the uncompressed sizes of all the RRs in the Answer section of the Zone transfer.  This should be more repeatable between implementations and reflects the actual size of the Zones. D-Root’s RSSAC002 tracking code is keeping track of that value as an alternate Zone size metric (we are also tracking the counts of each RR Class/Type).