2 tblsp is what people say most chins eat in a day, so to not waste food (assuming you're dumping the extras out each night) you put about that much in.
I just know what my chins eat in a day. Mei and Kona will eat their whole coop cup full, so I fill it up, Chibi eats one scoop (I believe it's a 1/4 c scoop) and MuShu eats the least pellets at just under one scoop a day.
So, if you aren't sure what your chins eat, put some out and make sure they have enough for the full day. Then after you get an idea of how much they eat each day you can scoop out the appropriate amount.
It was never recommended to only give your chins 2 tblsp and that's it. It was just a general guideline for how much on average they eat. Chins won't over eat.