I agree with @WhiteNoise that this doesn’t seem undesireable.
With that said, I’m not seeing what you’re seeing in the default css.
I don’t know where documentation exists for creating custom.css files, but by looking at obsdn dark rmx, I would guess it is set somewhere via .markdown-embed or internal-embed - perhaps a scaling factor.
Troll on through your obsidian.css and you may be able to adjust that setting.
EDIT: Untitled and Untitled 1 have the same contents in the example above.