Report Builder Issues For Large Reports

I have a very simple report model with two tables in it but having millions of rows.
I have developed a report on this report model, which will return atleast half million rows.

I have published this report to report server and trying to access the report through the report manager. It never returns the result. Fails with tme out expired error. I have 64 bit machine and 2 GB RAM. The report is expected to return atleast 500 MB data. As per the SSRS documentation it the memory should be three times to process the report. It does not work.

If I develop the same report using the report designer, I get the results returnd within a minute.
If I use the above mentioned Report Model as my data source, again I get the results within less than minute.

So what is it the report builder report is doing additional? It is strange that the same report model when used as data source returns the same number of rows within less than minute and the report developed and uploaded to report manager using report builder fails with time out expired error?

What is that the Report builder doing internally? Can I change some configuration for report builder report to work for large reports? I have already changed the memory settings in the report config and machine config files.
Your early response is appreciated.

Virendra