let's say one had a pair of 40s set up for a specific engine - what would happn if they were fitted to an engine of larger capacity (25 % )?

Will the mixture start to get weak under heavy loads or be weak for all loads?

I would say that the carbs would be about right as the volume of air dictates the mixture. Probably close enough to do some final tweaking on the rolling road anyway.

you would likely have issues with it being lean near the top end as the jets wouldn't be able to provide the amount of fuel required by the larger engine. I reckon it would run though and pick up fine.

That is of course if both engines have standard heads and standard cams. Put them into the mix and it gets a little more complicated to give you an answer.

Agree - Harsh reality is every engine is different.....I would say the chokes may need replacing, idle jets, main jets, Basically, to get them right, you would need a complete RR retune.....