If you feel the need to make the content on your in-game Pip-Boy larger, you can do that with a simple press of a button.
If you want to enlarge your Pip-Boy to better see something on the map or the stats on an item, all you need to do is press the button that's bound to Toggle POV. On the Xbox One, that's going to be the select button (the button with the two squares). Check youe controller settings if you're unsure.
If you press Toggle POV with the Pip Boy open, it'll make everything bigger. The text, icons on the mini map, everything. To decrease the size of the Pip Boy, press Toggle POV again and the Pip Boy will return to its normal size.