Double-clicking an email in the message list (the left column) creates a separate window for that email. While this separate window is open, if you minimize the main client, the email contents in the separate window suddenly become bigger. When the main client is open again (brought to front), the size of those in the separate window gets restored. FYI, while reproducing this behavior, I didn’t adjust any value in the zoom option.
The problem is shown for v10 clients, both of v10.0.3125 and v10.0.3117, when the Windows hi-dpi setting is used (i.e., “Scale” in Display setting in Windows is larger than 100%). If the Scale is set to be 100% or the client is v9.2, this problem is not observed at all.
One minor thing I’ve noticed after updating the client to v10 is that certain fonts used in emails look a bit blurred, which is not observed on v9.2. So I guess v10 may have some glitches related to the Windows hi-dpi functionality.