To me, it looks like new emails are viewed in a kind of “fit to page” mode. The entire email is shown and as the email gets larger, the text and pictures get smaller. In this mode, it is ignoring the read font size in settings. I have looked through the settings and do not see a view mode for read messages. I think that once you zoom it, it retains that font size for that message. With most emails over a page, this means I have to zoom in on every email.
Plain text emails should respect the preferred font setting in Tools>Settings>Mail>Preferred style.
HTML emails will use the font size designed by the sender.
If you use CTRL button and the scroll mouse button to zoom, that zoom carries itself out on each next message you read.
To return to a default view hold CTRL and 0.