"Nudge" says more people means more eating.

http://ajcn.nutrition.org/content/90/2/282.full.pdf

http://devpsychopathologyru.nl/wp-content/uploads/2012/12/2009-Modeling-of-palatable-food-intake.-The-influence-of-quality-of-social-interaction.pdf

http://europepmc.org/articles/PMC2276563/reload=0;jsessionid=pcefhmQohIp1CPVmEpqG.2

https://www.amherst.edu/media/view/231463/original/Vartanian%252Bet%252Bal%252B%2525282008%252529.pdf

This article suggests it's more complicated:

http://www.ncbi.nlm.nih.gov/pubmed/14599286

It's also believed that obesity is contagious across social networks:

http://dash.harvard.edu/bitstream/handle/1/3710802/Christakis_SpreadofObesity.pdf?sequence=2

But this article suggests otherwise:

http://medicine.yale.edu/labs/fletcher/www/ccf.pdf