I'd be surprised if it was an issue related to real capacity as opposed to traffic shaping. This reads like the exact kind of thing that net neutrality is meant to prevent.
The real question is why Comcast decided to upgrade the connections? Not upgrading Cogent peering is a pretty defensible position [1], although maybe they didn't want to look that much worse than their competition.
[1] See the previous results of Cogent peering disputes, which is about half of all the highly publicized peering disputes.