There appears to be no weight restriction.
As far as size, "Soft-sided pet carriers can exceed these dimensions slightly because they are collapsible and able to fit underneath a seat without blocking the aisle."
It appears the dog is the one that needs the wiggle room: ". . . the animal must be able to stand up and turn around comfortably."
It appears you're right - thanks for the relief. I called an agent to book my dog a few weeks and they were very keen to have the weight of the dog+carrier and remind me of it having a limit (8kg came to mind). This is an international hop if that changes anything..? (appears not). I am not on any Lufthansa legs, although that may be where the 8kg number cropped up, as that's LH's limit.
Our corgi will have ample spin room, the carrier is quite big for him yet is to-the-centimeter sized per United's requirements.
Thanks again for clearing this up =)