Thought I would post a follow up in case someone 6 months from now has exactly the same question.
Bradford pretty much nailed it.
My Montbell UL parka sized medium, fits fine over a baselayer and thin fleece or thin puffy vest. But beyond that there is not a lot of room for more than this, and in particular trying to sleep in it with my arm under my head, and so forth, while not threatening to rip, would be constrictive. Basically what people would call "optimum" fit for warmth, with not too much room to spare for flexibility. So that is the baseline.
The Mirage size medium is in fact a bit roomier as you would want/expect. The sleeves are a bit longer as well. Also fits "perfect" for me in size medium (very glad I didn't up the size), but is a more relaxed fit suitable for more serious layering if needed. VERY happy with it.
So hope that help people figure out the right size for them.
The medium mirage I got weighed in at 13.0 oz. The UL parka is 9.3 oz. In warm weather the UL parka would be the no brainier because wearing anything more, even at the coldest part of the night sitting outside, would just be uncomfortable. On the other hand, for any situations where I would not ever be uncomfortable wearing the Mirage, it seems like I will now be faced with an interesting quandary - for extra weight equivalent to a little more than the energy bar I might eat to help keep warm just before getting into my bag, I could bring the Mirage instead, which is in a totally different warmth category. It is definitely not just a tweak up in warmth. In the high sierras, even in the summer, I've never been *too* warm at night above 10k feet, even in the summer. So even without using that fact to consciously take a lighter bag, this jacket is still going to be tugging at me at me. I think this is going to be the ultimate test if I am firmly committed to being light for light sake, or whether still have trace elements of the Epicurean left in me. OK, who am I fooling about that one. LOL
For what it is worth, I got the gunmetal version, but this fabric is a shade darker that the gunmetal on the UL parka.