re: If you owned a restaurant, how would you handle a request for a 1/2 sized entree

Restaurants went to a larger portion years back when they discovered that larger portions could justify higher prices.
The "sales per customer transaction" increased, the restaurant was happy....thus also increasing the "sales per sq ft"
The bill increased, so the tips became larger and the waiters/waitresses were happier.
If a restaurant served smaller portions, they will likely be asked to charge less, so all of the above would change AND their FIXED expenses % would rise as a % of revenues

It is not the restaurants job to restrain people from eating.

I often will eat half and take the other have home...depending on what I have ordered.

Having said this, some restaurants might still profit if they reduce the size by 20% and the price by only 10-15%