Actually upon further investigation, the 600×9999 image generation only takes place if both the GP Premium and MailPoet plugins are activated. I had all other plugins disabled.
Also if i enable all plugins except MailPoet or GP Premium the 600×9999 images aren’t generated.
So as soon as both of these plugins are activated at the same time the issue occurs, regardless of what else is active/deactivated.
Any ideas?
Cheers Tom