I am using a Lumia 635 with WinPhone 8.1. For some unknown reasons, the start app panel could not fit into my screen size. I would like to have my full size app panel back, but I don't know how to make it. The attachment is a screen shot of my current app panel and I will appreciate any help and suggestion.
It seems like you have activated the third start screen column "show more tiles" in settings -> start + theme. You can either fill that extra column with apps or deactivate that setting.