Let's just clarify things to make sure we are talking about the same thing. Turning your iPhone into a hotspot makes it an access point for your computers and other devices..
So if you wanted to download 1MB file from the Internet and you were on your home network (Wired or Wireless), that's the amount you download. If you use the iPhone hotspot, you are STILL only downloading 1MB.
However, the difference is that you are using your data allocation (if you any any) on your iPhone plan. So obviously if you are only using the iPhone on the Internet, its data usage is one thing, but the moment you make it a hotspot, you are definitely using more data because that's what you wanted when you enabled the hotspot.