I have not seen this mentioned yet...
Does everyone put the pad
IN the bivy? I do too but I am wondering why. I like to curl up and move around and I change positions all through the night. When the pad is in the bivy, it keeps me from curling up because the pad keeps the bivy from moving with your body - like a sleeping bag. Which brings be to the next point - if you use a very minimal quilt that is cut to fit the pad and attach to it, you cannot curl you legs and must sleep like a board. That's why I like generous quilts with real foot boxes but those are essentially sleeping bags that are slept in open. That's why I use sleeping bags like quilts unless it is super cold.
If I use a sleeping bag like a quilt in a bivy and put that whole shebang
ON the pad, I get the best of everything. Either way (IN or ON), for my regular size xtherm still feels too narrow when I curl up and go over the sides, feeling like I am falling off the edge when I side-sleep. It leaves me wanting a wider pad because I felt "high centered". One night, I simply let out about 1/3 of the air. It allowed the xtherm to conform to my body more and it felt softer. It also removed the "high up" feeling and made it more comfortable in all positions including side sleeping. In temps going down to about 19°F, I have slept in an xtherm filled up only about 1/2 of the way with air and have been completely toasty because most of the R rating is from the reflective material that does not need the air. At the same time, I have frozen my azzz on an xtherm at only 28°F because I had set up in the dark and laid the thing down upside down - the reflective properties seem to be directional... "SHINY SIDE UP!"
When going to very lightweight quilts for slightly warmer temps, I am finding that they are TOO LIGHT in weight - not insulation. Because they are too light weight, they do not drape properly and I get pockets of big air that makes you feel cold. It is more of a chore to tuck edges in and for them to stay put. There's a strange comfort in a blanket/quilt/bag that has some weight to it. It is instinctive from childhood in wanting to feel swaddled for security and comfort. Unless a quilt has a good foot box and strategic cuts and tightening bungees, I find it hard to perfectly tuck in as easily as I can with just an opened up sleeping bag used as a quilt. I use a sleeping bag.
YMMV