block editor – WordPress giving size-large class to post images automatically as default (even if large size is unavailable)

I discovered weird wordpress behavior – wordpress automatically adds size-large class to newly added post images.
Even if image is too small and big size wasn’t generated by wordpress it gives that size-large class.

For example right now I have image which is too small even for “thumbnail” and “medium” size to be generated (only “Full size” is available in image size dropdown in gutenberg post editor, which is actually smaller than “thumbnail”) and wordpress gives it size-large class anyway.

Seems like this weird behaviour occurs when image is too small for certain sizes to be generated.
How can I solve this problem? “Full size” should be the option that always works anyway.

Right now I have “thumbnail” size set to be 850px wide, uploaded picture 800px wide so thumbnail and any other default wordpress sizes weren’t generated. The image has only full-size option available, but class is set to size-large anyway… I’m sure it’s not plugin related problem.

What the… ?