There are a few possible reasons for this. Firstly, it's possible that you may have your AirPods set to a lower volume than what you'd normally prefer outside. You can easily adjust your AirPods' volume settings from the Settings app on your device. Secondly, the dirt and debris that accumulate in the AirPods can also affect sound leakage. Cleaning your AirPods regularly is recommended to ensure optimal sound quality.