Hey Mandy - Unfortunately the mobile view and the desktop view are not COMPLETELY separate, I'd describe them more as *mostly independent* - but they are still siblings of each other. So if you change a font for example from H2 to H3 on mobile, it will also change on desktop. But if you change the position of the font inside the block (ie vertically centered versus vertically aligned to top), this can be independent. I have not found a definitive list of what changes are independent and what are tied to each other.
You can achieve the change you are looking at through CSS code, setting font sizes specifically for mobile over desktop.