Why does my font size jump around (large and small) when writing a post in Facebook for Android?

Pretty much the title. When I'm composing a post on the Facebook app for Android Galaxy 7, the font size keeps jumping back and forth between large and smaller until I hit about 30-40 characters. It's incredibly irritating and it didn't always do this. That stupid color wheel I can make smaller, but this font thing is driving me crazy.

I can't find a setting or option to make it stop doing that.