The one with a built-in USB port has an 8 Amp output.
The one without has a 7.5 Amp output.
Both are the same price

Both are the same length, so how did they keep the capacity the same with less internal volume for the battery portion? How does it have a higher output?

Fenix 21700 Rechargeable Battery - 5000U <<-- WITH USB PORT
SKU:
FX-ARBL215000U


Fenix Rechargeable 21700 Li-ion Battery - 5000 mAh

SKU:
FX-ARBL215000