It's because of limited RAM that this distinction started.

On especially older phones if I were to write a long comment and move to a different tab or app before submitting, I can all but guarantee the OS would kill and try reloading the tab and lose all my text. What's even worse is this could happen mid online purchase which can have even greater consequences (double booking or purchasing especially but things like flight tickets). People who grew up with older phones saw this happen all too often and moved to a desktop or laptop computer where that literally never happens, at least by default.

This, I'd bet, is the primary reason for big vs small screen activities, although of course there are many secondary ones, such as the phone being many kids' primary interface

This still happens on Android phones with enough RAM, it drives me insane and Firefox is especially bad for this since it will literally always reload the current tab when moving back to it. Phone software is just horrible all around. Multi-tasking simply does not work on phones.