> should Comcast now give my and my hoster free access

If your hosting company has paid Comcast for 100Mbps transit, then Comcast should honor that agreement with your hosting company. Your uplink to your hosting provider is completely irrelevant as far as Comcast is concerned.

If you're using a different transit provider (e.g., Cogent), then Cogent and Comcast will have a mutual peering agreement that accounts for imbalances in transfer. These details are very complicated, proprietary, and you should never need to care about them. That's the whole point of the internet; you connect to "the internet" and can treat it as a homogeneous network, whereas in reality it is actually a bunch of mutually-peered subnetworks.