Teardown of the Apple Watch Series 6 shows the 40mm model has an 8.5% bigger battery than the Series 5 and the 44mm model has a 3.5% larger battery (iFixit)

iFixit:
Teardown of the Apple Watch Series 6 shows the 40mm model has an 8.5% bigger battery than the Series 5 and the 44mm model has a 3.5% larger battery  —  Introduction The Apple Watch Series 6 is here to take your breath away—or at least tell you a whole lot about your breathing.

via Techmeme