Looking at Diamond lights on this site, the 7' light is listed as using 4 - 32 watt double pin bulbs (F32-T8/SP41/ECO), while the 9' uses 4 - 59 watt single pin bulbs (F096/741/ECO).
Googling those codes tells me that the 7' light uses four 4' bulbs, and the 9' light uses four 8' bulbs. Is...