From the docs in GitHub: "The size of the dots corresponds to the number of reviewed papers for literature reviews (non-reviews have the smallest size)..."

I wondered the same. It doesn't seem to correlate to P-Size, citations, or participants. Maybe a combined factor of each that is calculated? I'm really not sure.

Couldn't quite work that out either. I initially thought it might have been to do with the number of citations but that didn't pan out

I think it might be the number of papers used to answer the question. Not totally certain though