The 'Primary' earbud that is paired directly with the mobile device will always use more battery than the 'secondary earbud which is paired with the primary. This is not something to worry about and is perfectly normal. Swapping the primary between left and right occasionally will ensure that one earbud does not deplete at a faster rate than the other.
The battery life of the earbuds will vary slightly depending on the type of Bluetooth device you are using them with. Using AptX or AAC may deplete the battery quicker as more data is being transferred between your device and earbuds. If you can restrict your device to SBC only, you may experience a longer listening time. The volume levels of the earbuds will also affect battery life. Higher volumes require more power to drive the speaker inside. Reducing to a lower level can prove beneficial for your hearing and extend the use of the earbud.