The phrase "capacity battery" refers to the amount of energy that a battery can store, which determines how long it can power a device before needing to be recharged. It basically measures how much "fuel" the battery can hold. Full definition
This phone is backed by a 3,000 mAh capacity battery with fast charging support, which charges the device 60 % in just 37 minutes. (androidadvices.com)
With high capacity batteries being available today, a phone doesn't necessarily have to be bulky. (phoneradar.com)
Now it's unclear why the Honor 9 would ditch the headphone jack, but it could be to add a larger capacity battery in the Honor 9. (androidheadlines.com)