I've noticed it in big and small companies alike. I think it's b/c when someone is checking email his/her mind is not in the proper state to pause and type a detailed reply to a 10 paragraph message.
It's purely a consequence of all the demands on peoples' attention and the ambiguity with which most people write/think. Face to face or even telephone interaction allows for ongoing verification of mutual understanding. For an example of how long and detailed emails sent/read by smart, capable people can create much confusion, ill-will and chaos, check out LKML :)
