https://www.cnet.com/news/iphone-xs-xs-max-show-off-battery-origami-in-ifixit-teardown/

The XS Max has a two-cell battery (left), while the XS has a new single-cell one (right)

However, it highlighted one major difference in Apple's "battery origami." The iPhone XS, the smaller of the two new models, includes a 10.13 Wh battery (2,659 mAh), which iFixit noted is slightly downgraded from the 10.35 Wh one found in the 2017 phone.