With OBS, you start RTMP streaming with fixed resolution 720p.
With WebRTC as RTMP. you can start WebRTC streaming with resolution lower than 720p, depending on your channel bandwith, for example 360p, then it raise to 720p. It's WebRTC standart behaviour to adjust stream resolution on the fly. Seems like Facebook and Youtube adjust quality controls by first frame.
So you should use
REST API to publish RTMP stream to Youtube and Facebook rather than JavaScript API as described in
this example. In this case you start RTMP streaming when WebRCT stream is already published and probably has resolution 720p as needed (depending on channel bandwidth and quality).
Also you can try to publish WebRTC over TCP as described
here, but this may not escape resolution adjusting.