法華義疏 5.144

5.144

佛壽長短無有定相，但穢土感佛業弱故佛壽短，感生死業強故人壽長；淨土感佛業強故佛壽長，感生死業弱故人壽短。

The length of a Buddha's lifespan has no fixed characteristic. It is simply that in a defiled land the karma that evokes a Buddha is weak, so the Buddha's lifespan is short, while the karma evoking saṃsāra is strong, so human lifespan is long; in a pure land, the karma evoking a Buddha is strong, so the Buddha's lifespan is long, while the karma evoking saṃsāra is weak, so human lifespan is short.