I was pondering this same question in a Facebook group the other day. I had the theory that maybe it worked like compressing more information into a smaller space. (i.e. Zipped folders in Windows etc.) That depends on whether I'm correct in that depending on the size of the metalmind it can only hold so much investiture. (Like a bracer holds way more than a ring)
Maybe burning a coppermind lets you put more information into a smaller coppermind. Like if you could swallow a golf ball sized piece of copper that was full you could then pour the memories stored there into a smaller form, like a ring, or marble sized copper ball. That was just a spitball idea I thought of, though the above seems more useful/more interesting.