It seems like portable chargers are getting smaller, cheaper, with a larger capacity all the time. What better example of this phenomenon than this Omaker external battery, which boasts 15,600 mAh and two USB output ports that push 2.1A, the amperage needed to charge tablets and some phones at faster than a snail's pace. The device itself supports 2.1A input too, so it should refuel reasonably quickly once you've drained it.
For reference, the new Samsung Galaxy S6 will have a 2550 mAh battery. HTC's upcoming M9 will have 2840 mAh. You can do the math: you're looking at about 5 full charges for one of these flagships before this portable charger is empty, and that's a conservative estimate. The device is 6.5 inches long and 2.5 inches wide, with a thickness of 1 inch. For a battery of this capacity, its physical size is really quite small.
This Omaker model was already dirt cheap at $29.99 and with the promo code 22NUA55F, you'll get an extra $5 off. The 86 customer reviews at Amazon average a 4.7/5 rating, so it does not look like Omaker has sacrificed quality to get the price down.