I am not sure why the XA2 is considered as a much larger phone than the Nexus 5. I just have compared them together and the XA2 is only a few millimeters taller than the Nexus 5. That is not a big difference.
The pro for the XA2 is the newer aliendalvik that helps to substitute apps with android ones where there is no native sailfish version.
The pro for the Nexus 5 is the multiboot option which is priceless!

However, if you need to use any video calling or banking apps than I suggest the XA2.

On My Nexus 5 (because of the old aliendalvik) more and more android apps stopped/stops working. Use the websites instead of apps you'd say. Well, the native browser doesn't work well with many websites and the android version of Firefox crashes too often. (I have tried many other browser but some of them do not even install because of the old aliendalvik, others just crash often.)
This is how I see and how I experienced it.