Images can’t be enlarged if they’re less than a site’s thumbnail settings. I have two images – the first one is ~1000x500, and the second one is ~500x100. I’ve scaled each one down so their width is 200 below:
I’m able to hover over the first one to enlarge it, while the second image doesn’t allow me to do that because it’s smaller than the site setting. Enlarging should be enabled whenever an embedded image is larger than the size it is in the post, as opposed to having to be over a certain size.