No need to imagine; we used to be able to buy a second battery before manufactures started gluing them in.

Even in the lowest power consuming of sleep modes volatile RAM still needs to be constantly refreshed, so considering a device with a duty cycle like that of a phone or tablet you might find the power being used for memory is more significant than you think. But that refresh is essentially a fixed cost from a software PoV, if the memory is not used it'll still be refreshed, where chunks of modern CPUs can be almost completely turned off when not needed. There are secondary costs of extra memory use though (CPU to populate & later read, IO subsystem activity as there is less free memory for cache, etc.).

Don't forget age. Battery age is gonna have much more of an effect than subtle hw configuration differences.

Almost. One's a Toshiba X30 and the other is an X40. The CPU differs, but it's powered off in S3 so I am presuming that doesn't matter. Both are the same age as well.