One of my biggest pet peeves about ESRI is that it constantly takes existing functionality and removes it in subsequent versions. In the 3.X API, infoWindow.Resize(width,height) could be used to adjust the size of the popup according to need. From what I can tell, this isn't really possible in 4.X, except for trying to hack it by modifying the ESRI popup CSS classes (which has not worked for me).
Not all maps must be mobile friendly. It's perfectly fine to have apps that are built for the sole purpose of using it with desktop PCs. Sometimes, it's better to have more room to present information back.
Restore the resize.