I don't know what you are talking about. Anything less than a 1/2lb of beef is a normal to small burger. It isn't big until its over 3/4lb. And if you ordered a burger that size you better eat it all in one sitting.
But seriously, in my family if it was on your plate you were expected to eat it. Now whenever I eat, with the rare exception of really bad food, I finish everything that I made or ordered. While in Japan, several times when I went out with Japanese people the all were willing to leave food unfishisehd. So I would say your observations don't come down to national cultural norms but family level norms.