Win Explorer opens instead of default file manager

For a few months, I'm guessing since around when Win 8.1 came out, "Save As" and the Browse file location icon have been opening Windows Explorer with some programs instead of my default file manger, XYplorer (default by the latter's settings). And the Windows Explorer window that does open is full screen without any size adjusting facility, that is, neither in the top right corner or by dragging the sides (the arrows are there but there's no drag). Two programs I use frequently that are exhibiting this are Internet Download Mgr and PDF X-Change Viewer. This wasn't a problem before recently under Win 8 or Win 7. Any ideas? Did MS make some changes in this regard with 8.1? (Win 8.1 Pro x64). Thanks,

Windows Explorer is the default File Manager for Windows.

Except when it's not - I take it you don't use another file manager. In fact, Win Explorer can be largely overridden. I've got a number of drive and directory (folder) icons on my desktop that open in XYplorer. Attached are two screenshots showing 1) the XYplorer default file manager option and 2) how the Win Explorer window that has been opening with Save As and browse since around December doesn't have any size controls (example from PDF-XChange Viewer).