Yes, we have both early 2013 base models, 13" is 10-15c cooler. Tested it on the weekend just general browsing here. And that was forcing my 15" to discrete only, otherwise it would have been 15-20c more. My next one will be a 13". 15" gets too hot for sitting on your lap.
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.