[MlMt] Opening a received message is only half-size

[Dave C]

Up until recently, when I opened up a message in my Inbox I received, it 
would take up the entire screen. this is the way I like it and the way 
it was.

Recently this changes such that when I open a message it takes up the 
bottom half of the screen.

I can resize the message, and that “sticks” (closing it and opening 
it again retains the new size), but the next message is still reduced 
size.

Any suggestions what settings to look at to revert this issue?

Thanks,
Dave

MM 1.12.4
MacOS 10.12.6 Sierra