OK, I figured out what is causing this to happen. In my PC’s configuration for Screen Resolution, there is a choice for “Make text and other items larger or smaller”:
That configuration screen was set to “Medium - 125%”:
When I changed this back to “Smaller - 100% (default)”, the TDP window no longer increased in size on relaunch. So this is the setting that caused the problem with Toad. The “Medium” choice (or the “Larger” choice) causes the Toad window to get larger each time.
I really want to keep the “Medium” choice because it improves the readability of everything on my screen. For now I will just live with the window size growing with each relaunch, and perhaps this issue could be treated as a bug or an enhancement so that the TDP window size can stay the same no matter which setting a user chooses?