There is non-economy reason behind it i.e. “solidarity” by the sellers for the customers.
Padang food sellers mostly live outside their origin place: West Sumatra province — whose capital city is Padang. As migrants, they know well how hard it is to be ‘alone’, far away from family or such.
They assume those who take the food home are like (some of) themselves. No time for cooking, no big bugdet etc.
By giving extra sized rice (=nasi), the sellers expect the customer’s other members at home/rent can also eat the Nasi Padang together to save more money.
This tradition has a long history and it’s still applied till today.
Some says, in the Dutch colonization era, those who eat at the restorant were usually Dutch or Chinese or Arab ethnic, while local people, usually low classed labours, took the food home.
Even though nowadays the situation is not so identical as the former time’s, the solidarity spirit still lives on.