The biggest problem isn't sizing or font sizes, most designs stretch and scale and deal fairly well with that like even if you kick it old-school and use tables.
The big problem is catering to different input methods. A mouse has far greater accuracy than a touch display. You can put two links mere pixels apart on a desktop interface and that is fine because a mouse is more than accurate enough. The smallest interactive element a keyboard-and-mouse user can hit is probably about the size of a single period in a font. There are other issues with doing that, I'm trying to highlight the sort of accuracy you have with a mouse. A mobile user could never hit such a small target.
On PC, you can enter text and show the full content of the website at the same time. You can search in the page with a keypress. You can open multiple web pages at the same time. That is not possible on mobile.
Yes, although clicking links on a page with mere pixels between them is still a PITA for many users. I was reminded of this using the super cheap mouse and acquaintance was using on a dinning table, and getting precision was possible but really frustrating. And they didn't like trackpads.
As more and more countries have aging population I'd expect these kind of accessibility issues to be more prominent. Sometimes I feel like using modes (vim style) could help, with the user getting different tradeoffs when "reading" and "manipulating", if there was an easy enough way to switch between one mode and the other.
The big problem is catering to different input methods. A mouse has far greater accuracy than a touch display. You can put two links mere pixels apart on a desktop interface and that is fine because a mouse is more than accurate enough. The smallest interactive element a keyboard-and-mouse user can hit is probably about the size of a single period in a font. There are other issues with doing that, I'm trying to highlight the sort of accuracy you have with a mouse. A mobile user could never hit such a small target.
On PC, you can enter text and show the full content of the website at the same time. You can search in the page with a keypress. You can open multiple web pages at the same time. That is not possible on mobile.