So as you can see, the segwit inputs consume less weight and are thus cheaper to spend.
I know, but I was thinking in terms of actual blockchain size increase, not fees.
What's the input size for non-Segwit P2SH?
Note that messages aren't actually signed with an address.
I know, but that's how it's exposed in Core. The current lack of this feature is one reason I can see for some people still wanting to use P2PKH addresses.