Mac Mail automatically resizes windows sometimes...bug?

Thanks in advance for the help. I set my email window size the way I like and get tons of emails and I open and close them with no problem and the email window stays the same size, but every once in awhile, I get an email and when I open it, the window automatically resizes to a smaller size which causes all subsequent emails to stay that size until I stretch out the window again to the size I like...it is driving me nuts...is it a bug of some kind or is there a setting I can change so the window size always stays the same in Mail?