This is how the internals of a Lithium Polymer battery looks



With that being said, the battery manufacturer determines the tolerances. The plates and how they transfer the power through the tabs is all part of this process. So how they manufacture the batteries determines the internal (inside the battery) resistance. Now the mah or milliamp hour of the battery is determined through the size of those plates. So an 8000mah battery can have the same internal resistance of a 200mah battery and vice versa. The size of that plate creating the larger milliamp hour does not increase voltage. They merely act as a larger fuel tank in the simplistic nature. Danny of SMC discusses this a lot on his facebook. He honestly is one of the most beneficial people to the industry. For stock a lower resistance pack will allow for more power to be transferred into the esc to be converted to the power needed for the motor.

Higher resistance = smoother (makes electronics work harder)
Lower resistance = punchier (makes electronics work easier, creating more power)

Higher mah = more run time
lower mah = less run time but possibly lighter (LRP sells these as lcg packs)

What I run in my cars:
Stock buggy 4250
Stock stadium truck 5400
Stock SCT 5400

I have ran the 4250 in all my vehicles until the 5400 have come out. I can make run time with the 4250 in stock sct (probably the most load pulling class raced at big races) but I can feel the discharge curve hitting its limit at eight minutes after two minutes of warm-up laps.