I'd assume that running a model that only needs to deal with a single programming language (the Copilot plugin knows what kind of code base it is working on) is _a lot_ cheaper than running the "full" ChatGPT 4.
This is what shouldn't add up: Microsoft is literally adding GPT-4, for free, to the Windows 11 taskbar. Can you imagine how much that costs when you look at the GPT-4 API, or ChatGPT's subscription price? Either Microsoft is burning money, or OpenAI agreed to burn money with them. But why would they do that, when that would compromise $20/mo. subscription sales?
Something doesn't financially add up there.