The MC window resizing behavior is WEIRD. I can't explain exactly what it does or why. Your problem is new to me, so I tried to recreated it.
I was able to resize the window exactly as I wanted 4 or 5 times. But then I tried doing it *faster*. I grabbed an edge, moved the mouse, and then VERY quickly released the mouse button.... and it did what your video shows! If you try to resize really quickly, MC snaps back to it's original size and shape.
So, try doing it slower. Go to an edge, hold down the mouse button, and drag to a new place while holding. Now slowly letting go of the mouse button. You might even wait for about 1 second before you release the mouse. It should stay at the size you dragged to.
I run with MC maximized almost 100% of the time. You can easily maximize MC by double clicking on the title bar at the top.
Good luck.
Brian.