The No. 1 rule when consuming sake with others is that you are never supposed to pour your own glass. It’s a serving tradition in Japan known as shaku suru or kumu, and it’s a way to show your guests you honor and respect them. In a setting where there may not technically be a host, the elder person should have the sake served to them first before the younger person or people in the group get their cups filled.
In addition, when hosting, you want to continually show respect and generosity to your guests. The smaller the cup, the more often you can pour for them, thereby showing honor and reverence each time they drain their glass. This is the main reason traditional sake cups are so small, to allow for this ritual to take place as much as possible when drinking with friends.