The "default" mtu of 576 is because, well, 2400 baud signaling is pretty
darn slow and interactive performance (or any kind of multileaving of more
than a single connection packet stream) is, what do we call it, laggy.
Sort of like trying to telnet while doing a bulk transfer if you have
bloated buffers, and do not use a decent QoS scheduler -- only with echo
times in the order of seconds per character.
I believe LCP uses a signed two-octet integer for frame size negotiation,
so you can negotiate quite large frames if you so desire and so configure
your endpoints.