I wasn't exclusively considering iPhone, because I also have a few friends now with Samsung phones (Galaxy 2 and Galaxy 3). These smartphone wielding friends all have kids, so that might explain the extra video/photos usage. And also corporate email with plenty of attachments. I still suspect that takes up an insignificant fraction of the space used by photos and video (and music) however.

In my mom's case, I drew my hypothesis by her digital camera usage. However, with that device I've been able to control the lack of sync (on her part) with an Eye-Fi card that I've pre-set with their endless memory feature and backup to her computer and my FTP site. Otherwise I'm sure she'd need an army of SD cards.