A bigger case on a laptop usually (but not always) means better airflow and therefore cooling. It might also mean there's room for bigger fans here and there, but without seeing inside it, it's impossible to tell.
I seem to recall a post where someone said PCS had told them the 17.3" chassis has an additional fan and will cool marginally better.
Needless to say it was a while ago and I can't find the link atm...
Edit: Found it https://www.pcspecialist.co.uk/foru...rms-of-cooling&p=370966&viewfull=1#post370966
Turns out this comment was for the Optimus though, so may not apply to the Defiance and Octanes. That said, someone will probably have posted pictures of their 15.6" and 17.3" Pascal-bearing clevos with the backs taken off on the web somewhere.