The smaller one is written in Spanish. Maybe the other side is English?
I know that McDonald's alters their sizes for other markets. Maybe Costo was selling Tang that was made for the Mexican market? I know they sell Mexican Coke.
Also somewhat interesting is that the new can has the spanish phrasing in front of the english. Could it be that he received the "latin american" version which has always been more diluted? Sometimes suppliers will run out of the "american" version and end up shipping product meant for other markets.