InDesign files jump to a larger window size after being open for a few seconds

Hi all, I'm using InDesign 17.3 on a 2019 iMac running MacOS Catalina 10.15.7. What's happening: I open an InDesign file and it remembers the last window size used. Then, an instant later, the InDesign window size increases to a size larger than my Mac monitor, so part of the window is hidden bottom and right. Before (1) and After (2) screen shots are below. Thanks in advance for any help!