(At first I thought you were asking about MQTT-over-websockets, but you're actually asking about MQTT vs websockets.) I believe MQTT implementations tend to be

(At first I thought you were asking about MQTT-over-websockets, but you're actually asking about MQTT vs websockets.) I believe MQTT implementations tend to be smaller, which is why it's popular for IoT devices. Both HTTP and WebSockets are much more diverse specs, so a generally available implementation might have a lot of features the device never uses but still occupy disk and memory (multiple kinds of headers and their semantics, data encodings, etc).