Text Size in Threaded Messages Will Not Change

So I recently changed my text messages from "classic" to "threaded" and the messages now randomly increase in size.

I know that I can change text size by going to Settings>System Tab>Screen>Text Size - and I've done this a few times to change the font back to a smaller size... but why is the text size increasing randomly in the first place?

For example, I'll send a message, put the phone down, and when I pick it up again the font will be huge. So I'll go to the settings to change the font size, but it will be exactly where I left it before (on the second smallest setting). I actually have to change it to a larger size, then change it back to a smaller size to get anything to happen.

Any ideas on how to make the text size stay at the size I set it to? Thanks!