iMacs are thin, but I'd hesitate to call them light. It takes a LOT of effort to push an iMac off a desk, especially a 5k model.
21lbs on a low surface area stand? I don't think I've ever built a computer that weighed <50lbs 😱. I'm sure it's designed to withstand some force from different directions, but children and pets have done damage to heavier things... especially when there is a power cable under the desk.