俱舍論記 11.114

11.114

「少光以上至所壽劫量」者，答。少光以上八十中劫大全為劫，於四劫中以大為劫。自下諸天大半為劫，於四劫中時等壞劫，即由此四十中劫為一劫，故說大梵王過梵輔天壽一劫半。謂大梵王以成、住、壞各二十中劫，總六十中劫為一劫半，故以大半四十中劫為下三天所壽劫量。問：大梵王望少光天，身量但狹半踰繕那，如何壽量懸格不同？解云：身、壽二量不可俱齊。為順火災故，大梵王不受大劫；為順水、風災故，少光已上大全為劫，倍倍漸增。

Regarding "From Parīttābha upward […] the kalpa measures of their lifespans": this is the answer. For Parīttābha and above, the full eighty intermediate kalpas constitute a kalpa; among the four kalpas, the great [kalpa] is taken as the kalpa. From there downward, for the various heavens, half of the great [kalpa] constitutes a kalpa, which among the four kalpas equals the time of the destruction kalpa; thus these forty intermediate kalpas constitute one kalpa. Therefore it is said that the Great Brahmā King exceeds the Brahmapurohita heaven's lifespan by one and a half kalpas. That is, the Great Brahmā King takes twenty intermediate kalpas each of formation, abiding, and destruction, totaling sixty intermediate kalpas as one and a half kalpas; hence half of the great [kalpa]—forty intermediate kalpas—is taken as the lifespan kalpa measure of the lower three heavens. Question: As for the Great Brahmā King compared with the Parīttābha heaven, his body measure is narrower by only half a yojana; why are their lifespans drastically different? Explanation: The two measures of body and lifespan cannot both be completely uniform. In order to conform to the fire calamity, the Great Brahmā King does not receive a great kalpa; in order to conform to the water and wind calamities, from Parīttābha upward the full great [kalpa] constitutes a kalpa, gradually doubling and redoubling.