In most cases, adding internal volume to a down sleep system will have detrimental effects on insulation efficiency. It’s usually an inefficiency feedback loop. Increased size promotes sprawl sleeping, which increases heat loss from the body. Then, that lost heat enters the system cavity, but the increased volume increases the lag time before that space can be heated by body heat. Last but certainly not least, the larger size has increased surface area exposed to the outer cold, which increases that rate of heat loss out of the system. So the body loses more heat, the cavity is slow to warm, and the outer surface loses the heat faster.
That said, it doesn’t always apply. There is an argument that added dimension in a sleep system doesn’t matter because the unused sections would collapse and close off the extra surface area. While there are some scenarios where this applies, it’s mostly short-sighted in the context of UL gear. Of course, this certainly applies to the big, heavy comforter on your bed at home. Something like that has enough weight to essentially press down and seal off the excess, leaving a small and warm cavity around your body. However, this is one of the reasons that UL down comforters don’t work that well. They are so light that they tend to float on top of you and not seal the edges. An open comforter really needs some weight to it to seal the warmth around you, unless you are going to actively manage it by tucking it around you.
The other place this theory could apply is with basic quilts. If a quilt lacks the technology to affectively manage drafts, the blunt method for dealing with them is to make the quilt wider and wider. This is where we run into the issue of quilt weights starting to click back up to the weight of sleeping bags. It’s a pretty thoughtless method instead of engineering intelligent systems to deal with drafts. However, this could be one place where added width doesn’t directly affect insulation efficiency. That extra width can just be tucked further around the user so that the internal cavity can remain small and efficient. Of course, it’s still a very blunt and inefficient method because you’re carrying the weight of all that width and there are much better ways of dealing with drafts. Adding width is simply throwing more weight at the problem. Additionally, this really only applies to the open top portion where the sleeper can use their hands to tuck it around them. The closed footbox is still going to remain cold if it is also sized large. The light weight of down construction is never anywhere near enough to actually collapse the excess volume and it’ll remain exposed surface area.