Spam tends to be a lot smaller (~6.4kb) as longer messages take more resources to send and are easier to detect. https://www.trustwave.com/support/labs/spam_statistics.asp

Also, Email does not need to hop from mail server to mail server all that much due to DNS. Granted legitimate mail might move around a fair bit, but as far as the public internet is concerned the vast majority of spam is sender -> possibly senders mail server -> spam detection software -> /dev/null.