Those of us who only drink small quantities of tea or coffee should use a smaller jug. The better the filling quantity fits into the container, the better the vacuum jug or flask can keep the beverage warm.
Which is why we have the appropriate jug size for all requirements – from the small two-cup variant to a pump-action jug that holds three litres.
The right preparation is also a factor: Hot beverages stay hot longer if you rinse out the vacuum jug with hot water before filling the beverage.
The same applies in the summer with cold water for refreshing beverages.