I have seen sockets with that 660W rating always thought it nuts.
Probably current limit of conductors within the socket, noit including
heating of the socket.

I USED to be able to buy a 300W bulb, today the largest I can find is
250W

We use CF now for nearly all applications so the heat isnt as
important. Although I still use a 250 watt bulb in my attic, it gets
very little use