Early 2013 I believe is not Haswell
I had a pre-Haswell rMBP 15" and sold it. One of the reasons was it got rather hot when doing nothing. It bugged me a little.
The new ones from all indications are much cooler running. I can't be sure about this as I don't own the new one (but will very soon), but this confuses the issue, I would think.
As you asked though, "All things being equal", it's probably a safe bet to say that across a particular chipset, the 13 will be somewhat cooler.
A lot of guess work....sorry 🙂