Yes you are, unless there is some prove that AW6 40mm for some reason have longer PRACTICAL battery life than AW6 44mm?No I’m not missing the point at all. You’re saying that your current Watch doesn’t reach 48 hours. Apple say that the expected battery life of a watch is 18 hours. They don’t state different battery ratings for the two different sizes.
As long as your watch is meeting apples expectations then it’s not faulty. If you really think you have a case then put a call into Apple but I bet they’ll tell you the same. If it’s lasting 18 hours then it’s fit for purpose
I know what Apple says and their figures are so low that I don't think it is easy to not reach that even with seriously faulty battery.