In markdown mode, all of the fenced code blocks and inline code literal strings are displaying with this very large font. The size is large, and also I'm pretty sure it isn't the same font as the default surrounding text (which is Classic Console).
This is Emacs 25.3 on a Mac, using the standalone app (the one from https://emacsformacosx.com/).
M-x customize-face
. If there is more than one thing happening at the particular location at issue, typeC-u C-x =
and read about what's there to determine what things you may wish to customize.