Upon examining the certified lab tested RF data and crunching some numbers, what is immediately apparent is that the iPhone XR has -- as expected -- dropped the 4x4 MIMO antenna design from the iPhone XS and XS Max in favor of a standard 2x2 MIMO antenna. This tradeoff of fewer antennas and MIMO channels, though, ostensibly has allowed for greater antenna gain and, concomitantly, higher radiated output power compared to those same metrics on the iPhone XS and XS Max.
Further analysis may be forthcoming, but for now, the graphs of the iPhone XR EIRP and antenna gain largely can speak for themselves.
For graphs and analysis of iPhone XS and XS Max lab tested figures, see my previous articles (1, 2, 3).
Source: FCC OET