Well, we don't really know the sales number this time to be sure, for all we know, Apple might have ordered much more 4.7" to manufacture hence resulting in shortage of the Plus. The Plus was also in shortage for the 6s series due to lower order..
The reason why the Plus appears or may be more popular this time is purely due to the fact that it has some real advantage compared to the 4.7" apart from the battery and OIS advantage. We see real differences like one more camera module and 1GB more RAM. Apple fans know how hard it is to have Apple actually include more RAM in their mobile devices.
Unless they manage to retain the form factor by increasing the screen to body ratio to retain some form of one handed usability, I highly doubt they'll create bigger screen devices then they already are.