The only difference between the 38mm and the 42mm is the bigger screen on the 42mm and slightly larger battery capacity on the 42mm. the 38mm has a 200mAh battery capacity. the 42mm would be around 250mAh or possibly 300mAh. Everything else is the same.
Battery should be bigger according to iFixit. But remember powering a 4 mm larger screen takes more energy also. So do we really see better battery life? Probably not
According to Apple, the "battery performance claims are based on test results from the 38 mm Apple Watch. A 42 mm Apple Watch typically experiences longer battery life." Take from the iFixit teardown
Battery should be bigger according to iFixit. But remember powering a 4 mm larger screen takes more energy also. So do we really see better battery life? Probably not