It won't be a must and I see in Roobet's art contest history, prize pools are different. I guess it depends on their assessment about important of each event which will convince them to allocate bigger or smaller prize pool.

Higher prize pool is more attractive especially in times of inflation but it is not a must requirement. Even with low prize pool from past contests, I see it is attractive enough. Additionally, they don't have much competitive contests in Bitcointalk.