This is done for the same reason as https://github.com/hashicorp/consul/pull/4005 and essentially reverts https://github.com/hashicorp/consul/pull/4953 which gave us unwanted unfurling behavior for links to consul.io from services such as Slack.
Currently the large image can load in Slack (or other clients that expand links) and dominate the viewport. This is just following the pattern from a recent change to the Terraform website in https://github.com/hashicorp/terraform-website/pull/214
Jeff Escalante