Let's use email as an analogy.

The body of the email is the actual transaction data (version, input, output, locktime). The email signature is the signature, proof you are you and you own the utxos mentioned in the body. You can also add a P.S. and it count as a part of the signature.

If emails had a cost to send, and they used the segwit discount pricing structure, the signature would cost 75% less than the body.

Now imagine you run mailchimp. How would you structure the emails you blast out? Well, you'd probably max out the signature...