Question about extracting subgraph G'

I've read the paper CGgraph and I've really learned a lot. It inspired me in several aspects. 

I have a question about extracting subgraph G' which will be located in the GPU global memory. After we get the vertex list ordered by in-degree descending, why do we perform a prefix-sum on `out-degree` to estimate the size of G'? 

Expect your response, thank you.