I noticed using v9.1.1 that if you put an image (jpg, png or gif) inside Content block (without image dimensions) and then resize the window smaller the image doesn’t downscale at all like it did with v8.
Using the Image block it downscales nicely, but I often use images inside main include and this is a problem if images will not downscale.
I ran across this in a package I’ve been working on. Ended up using some jquery to target the images and add the img-fluid class to them. For reference, here’s the code that solved the issue in my case: