If you look at email and think that you might have a tab open at home on a computer that it pops up in, that it might also push to your tablet and your phone and that if it is Gmail it might be mirrored on multiple servers, then add in a several kilobytes (or more) of images... I can definitely see it starting to add up to 3-4 grams.

Now factor in that excel sheet or word doc, some photos or a pdf that you're sending to a few (or hundreds) of coworkers...