is the battery in the 40 mm apple watch smaller than the battery in the 42 mm apple watch?
I want to know whether my wife's apple watch will stay charged longer than mine under same conditions.
Apple Watch Series 6, watchOS 9
Apple doesn't mention a difference in the tech specs:
Apple Watch Series 8 - Technical Specifications
However, unless both watches have exactly the same apps and you both do exactly the same things in the same places at the same time, it's almost impossible to make a direct comparison.