How much should we eat?

Since I came to London, How much I eat is dramatically decreased.
When I was in Japan, I worked as a sales person, so eating lunch with colleagues is a kind of a part of work, and usually went out to eat, but in London I usually eat in my place and the amount is not a lot. Sometimes I am really starving and feel bad, but I might get used to it, one day I felt good when I was heavily hungry.

Recently I have thought that you don’t need to eat a lot. Many advertisement and some TV programs recommend to have sufficient nutrition, but I think sometimes it is too much. It might kind of strategies of food industry.