Thanks to @Broadus for reminding about the somewhat different effect of Zoomed Display mode on the XR.
For all the phones that support Zoomed Display mode, all of them --- except the XR --- provide about the same font size. That means it doesn't matter if it's the iPhone 6, 6s, 7, 8, 6 Plus, 6s Plus, 7 Plus, 8 Plus, or XS Max, all of them have about the same size text in Zoomed Display mode.
But not the XR. In Zoomed Display mode it has smaller fonts than all of the above phones in Zoomed Display mode. OTOH, the XR in Zoomed Display mode has bigger fonts than all iPhones in Standard Display mode. IOW, font size of the XR in Zoomed Display mode is kind of medium sized. This makes sense, since the phone is effectively a smaller XS Max, so it stands to reason its Standard and Zoomed fonts are also smaller than the XS Max in the same modes.
Here is my post, but I'll repost it below to illustrate the math:
The iPhone 6s' Zoomed Display mode turns it into a giant iPhone 5.
The iPhone 6s Plus Zoomed Display mode turns it into a giant iPhone 6s.
The iPhone XR Zoomed Display mode turns it into a big iPhone XS.
The iPhone XS Max Zoomed Display mode turns it into a giant iPhone XS.
Remember, the smaller the points per inch, the bigger the fonts:
Screen height of 6s is 4.1". iPhone 5 vertical screen points is 568, so Zoomed Display is 138 points per inch.
Screen height of Plus is 4.8". iPhone 6s vertical screen points is 667, so Zoomed Display is 139 points per inch.
Screen height of XR is 5.5". iPhone XS vertical screen points is 812, so Zoomed Display is 148 points per inch.
Screen height of XS Max is 5.9". iPhone XS vertical screen points is 812, so Zoomed Display is 138 points per inch.
So for Zoomed Display mode:
5/SE: No Zoomed Display
6/7/8: 138
Plus: 139
XR: 148
XS: No Zoomed Display
XS Max: 138
For Standard Display mode:
5/SE: 163
6/7/8: 163
XR: 163
Plus: 154
XS: 153
XS Max: 153
This means that the 6/7/8, Plus, and Max all have about the same font sizes in Zoomed Display mode with the points per inch at about 138-139.
In native mode, the 5, 6/7/8, and XR are all the same at 163, and the Plus, XS, and XS Max are all about the same at 153-154.
The XR's Zoomed Display Mode is the odd one out at 148, a medium font size. This actually works out well for my wife. She wants bigger than native 163, but doesn't really need as big as 138, so 148 is good. In truth, that's only a little bit bigger than the native font sizes of the Plus, XS, and Max.
That was (and is) an extremely helpful post. One advantage of the XR in zoomed display is that it is not so very large as regular zoomed displays are rendered in some apps.