between the minimize button and the close program/window button in the top right hand corner, there's a + sign for sizing or maximizing a window. I suspect at some point you had both programs, using that choice button set on sized windows instead of maximized windows.
My computer using Windows 10 has suddenly developed an issue. Whereas before, I could easily switch from the internet to an MS Office window by clicking the mouse from one to the other, now it forces me to expand or minimize to go back and forth, which is more of a pain to have to do. How do I get it to go back to the previous easy method?