It's not the length of the message that is the problem in any way, at least with text. I mean, even taking into account bandwidth one doesn't care if a message is 5 bytes or 500 bytes. If it includes attachments, yeah it makes sense to penalize huge posts.
The size of a message is not a matter of bandwidth, but rather a matter of mind lazyness. If you read a long message just to realize at the end that it was just crap, then you can consider you got screwed. You would have prefer a shorter message.
Therefore short messages has to be rewarded. Thus, to me it make sense to pay more for longer messages.
Excessive, lazy quoting (like this message!) should also be discouraged.