Here is an interesting tidbit I picked up today that concerns the Report Viewer controls.

Mohammad Farooq, one of our fine support folks, noticed that the same report exported as PDF from our Report Viewer control was a different size depending on the rendering mode being used by the control.

When rendering a report from the server via the control, we got a svelte 300 KB file. However, when you put the control in local mode and exported, we got a 20+ MB PDF!

This behavior is due to the fact that PDF documents are (normally) compressed when they are generated. This compression logic lives on the server-side (inside the SSRS code), but not in the Report Viewer controls. So, if you use the Report Viewer controls to render a "server report", you enjoy the benefits of "server-side" compression. If you don't, you get no compression.