That's what 20 MB blocks require. That means, you receive the blockchain, that's 84 gigabytes per month. You send the blockchain to one peer, that's another 84 gigabytes per month.
There is only one time that one node has to upload or download all of the blockchain, and that is when a new full node comes online. It downloads the blockchain from one other node, and that is its only full blockchain download. If you don't want to be the node that has to upload the full blockchain, you can change that in your config file. The full blockchain download is definitely under the monthly caps. Those 84 gigabytes happens only once in the nodes lifetime, and the blockchain won't be 84 gigs even when 20 MB blocks are accepted. The 20 MB blocks just mean that the maximum block size accepted by the network is 20 MB, not that every block is 20 MB. Get your facts straight.