I think that's vaguely related. It would be reasonable for webrtc to
try to do path MTU discovery, which under IPv4 would mean setting the
"don't fragment" bit on sent packets, and respond to incoming ICMP.
(Which would then be broken if ICMP is gratuitously blocked, just like
TCP path MTU discovery breaks down in that case).
But we don't do anything like that, as far as I'm aware. Which means
that the IPv4 network is expected to fragment any packets that are too
large for some link on the path. Which has the drawback that if one of
the fragments is lost, other fragments will consume network resources
but still be useless if they make it to the receiver.
So the smaller max packet size is to avoid packet fragmentation.