You have accidentally zoomed your messages. Your text can be enlarged by pinching and zooming and you can also decrease the same way. I imagine you have figured this out by now, but thought I would respond in case you have not. This can be effective when you need to zoom in, but it is not convenient on a regular basis. We should not have to zoom in each time we open a message in order to read it or to see exactly what we are typing.
Of course, the typing on this phone is ridiculous as well. I keep getting random letters i.e. bbn, CVS, ing, AMD when typing. And it is not the screen protector. If I use Micorosft launcher or gboard the problem is remedied. We should not have to download a different launcher or keyboard in order to type efficiently on our phones.
If you notice, that will also change the font on your other screens and much of the text in your captions will be lost. They need to restore the messaging app to earlier capabilities.