BOINC is using about 25GB of disc space. It has available 100s more. How do I expand it, or can I?

What is the limiting factor, processor, video card, memory, disc ?

You can allow more disc space for BOINC (assuming you are running Windows) from the GUI - go swap to the "advanced" view if you aren't already using it. Then select the "options" menu, then "computing preferences" option, finally select the "Disk" tab - here you can set the "use no more than" to allow more space (there are a few options, which are self explanatory. Finally hit the "save" button.

How much of the allowed disk space BOINC, or rather projects running within the BOINC environment, use depends on the projects' applications you are running and how many tasks are being run.

The limiting factor is rarely disc space or disc speed, but any of processor, video card and memory can be, again it depends on the projects' applications being run.