Unpredictable Monospace text sizing esp inline images

Site URL: https://www.peachofmyart.com/work/vivid-magazine

I've been using the monospace text mostly for image captions & therefore the settings are all directed to it being very small. 
For some reason however when I have an image set to the 'poster' design style, the monospace text is huge. 
The first screenshot is an example of the size difference on desktop between monospace text in a poster image and monospace text in a textbox.

Is there a way that I can fix this? I know I can just change it to paragraph 4, or some other formatting option, but if possible I like having the image captions a different font to the other text elements on my page and would prefer not to do that if there's a not-too-much-harder fix.
Font/monospacing metadata settings below.

Can you share link to page where you use Poster BLock? WE can check easier.

I don't see poster block on above link.