Does the amount of caffeine in a tea bag decrease if larger water is used?

No, the amount of caffeine in a tea bag does not decrease if larger water is used.

The amount of caffeine extracted from a tea bag primarily depends on factors such as water temperature, steeping time, and the type of tea leaves used. Larger water volume might dilute the tea's concentration if added after the steeping process, but it doesn't affect the total amount of caffeine extracted unless more than one teabag is used, or the original bag is resteeped in the diluted liquid.