He's using smaller capacity cells. You can't look solely at individual cell capacity. You have to take in account the total watt hours of the battery. For instance, 4S6000mah and 6S4000mah setups should perform the same provided everything else is optimized for the battery (motor kv, wire size, connectors, etc.). Personally I would have like to have seen 3S become the standard in 1/10 making 6S (via series connection of your 1/10 packs) the standard for larger scale. That became the standard setup of the aircraft world immediately when lipo was introduced.