That calories are used as social lubricant irks me a lot. I understand why it was so in the past, but we live in a world filled to the brim with food, do we really need tens of thousands of calories at any social gathering?
The answer is obiously not, indeed it would be beneficial to lower the amount circulating... But as Lumifer spotted and wannabe rationalists often overlook, what appears as waste and irrationality is actually a situation optimized for status.
Ignoring status is almost always a bad idea, BUT: we can always treat it as just another contraint.
Given that we need to optimize for status and waste reduction, what could we do?

• coordinate with a charity to pick-up the leftovers
• use food that can be easily refrigerated and consumed gradually later
• have food in stages, so that variety masks lack of abundance (and pressure people into eating leftovers)
• repackage leftovers and offer them as parting gifts ...

These are just from a less than five minute brainstorming session, I'm sure someone invested in this would come up with much more interesting and creative ideas.