It's an interesting suggestion!!

No idea about the Digger accounts, but bullets make a difference and certainly in the old style round nose long heavy bullets less effective wounding would be likely. US Army testing cited by Genl Julian Hatcher in the pre-war era definitively demonstrated the ".256" {6.5} caliber to produce far more traumatic wounds in pig and other livestock testing. This with, IIRC, a 123 or so Spitzer bullet. Roy Dunlap more or less praised the 6.5 Jap caliber and Lt Col John George in his famous "Shots Fired in Anger" stated the Jap 6.5 was "entirely adequate in mankilling".

The move to the 7.7 in Jap service has been described being done for advantages in larger volume for tracing material and of course in the heavier bullet ball cartridge more effective penetration of "soft" obstacles {wood barriers, building materials, etc}.

George praises the Jap 6.5 for its ease of shooting and he himself carried a Jap carbine for a while. It's shootability alone might have claimed more lives than its more diminutive ballistics lost, if the latter actually occurred. He is dead right about the shootability. Almost zero recoil. We have a Jap 6.5 Arisaka and I used it to place very high against modern, topline AR's on the ARFCOM SHTF Challenge with shooting of 15 shots in 3 minutes, field positions-only, no prone, no sling at 100 meters in spite of the need to reload the 5-shot mag by hand {no clips} versus the AR's with no need to reload at all. As a combat weapon the Jap 6.5 would be hard to beat among the bolt guns. A Lee-Enfield with a 10 or 12 shot mag in 6.5 Jap or Carcano would be the Cat's Rat!!