Voltage is the power a battery can deliver.
That means if you have two 1.5 Volt batteries, and one is rated at 2000 mAh and the other is rated at 2500 mAh, the 2500 mAh battery lasts longer. That doesn't mean the higher capacity cell performs better. Batteries respond to loads differently. Memory cards and modems are massive loads to batteries. You may experience low voltage conditions with brand new alkaline batteries because they do not respond to power surges as well as Lithium, Nicad, and NiMH batteries.