You'll find that makes a nice "round" number in lbs... specifically 19,000 lbs... and likely related to some FAA based rule
A quick google seems to indicate that the FAA deems "small airplanes" in the "commuter category" to be 19,000lbs or less...
Originally Posted by https://www.faa.gov/aircraft/air_cert/design_approvals/small_airplanes/categories/
Commuter Category- Maximum takeoff weight of 19,000 lbs
- Maximum passenger seating capacity of 19
- Multiple engines
The CASA exemption you're talking about is: "CASA EX25/18 — Class 1 Medical Certificate (Exercise of the Privileges of Certain Flights by Holders of a Commercial Pilot Licence or Air Transport Pilot Licence) Exemption 2018"
CASA EX25/18 Exemptions as made
This instrument grants an exemption that permits the holder of a commercial pilot licence or an air transport pilot licence to exercise the privileges of the licence to conduct certain flights while holding a class 2 medical certificate instead of a class 1 medical certificate. The exemption in the instrument covers a flight if it is wholly within Australian territory, is in an aircraft with a maximum take-off weight less than 8 168 kg, and does not carry a passenger.
Basically, it means a CPL or ATPL can do commerical, non-pax ops in aircraft with a MTOW of up to 8618Kg with only a Class 2 medical... For instance, flying cargo in a Beech1900... But this does not mean that a Class 2 medical limits a pilot to aircraft with MTOW of <= 8618Kg... it only applies if they're intending to make use of this exemption.