The graph populated is taking about 190MB of memory, i.e., 32 bytes/ vertex approx. I was expecting it to take 4 bytes/vertex.
That does sound high... are you using a 32- or 64-bit platform? Each vertex needs to store a vector, which is typically 3 pointers (12 or 24 bytes, depending on platform).
I should have been specific about the machine that I was using. I used 64-bit platform. That explains 24 bytes out of 32 bytes. I am not clear about the remaining 8 bytes. It could be seen from the code that I posted, there are no edges in the graph at all - only vertices. The vectors used for storing the out edges should be empty. Maybe, instead of vecS as OutEdgeList, I should be using listS (or slistS) if number of out edges per vertex is less, say, 2 or 3.
The next release of the Graph library will have the compressed_sparse_row_graph graph type, which requires much less memory than adjacency_list. Naturally, it's also less flexible.
That's good news! I am eagerly looking for the next release. Thanks for your reply Doug. My doubts have been clarified. Regard, Pratyush