I think you are correct about the particle size... I Googled the key words from your answer and came up with this... although not specifically about pt printing it provides some insight (and some interesting information about Ag/Pt toning)
Section 4 states that the tone of the image depends on size, distribution & morphology of the particles... although the discussion relates primarily to gold and silver particles I am sure the same applies to platinum.
I particularly liked the illustration with the disembodied hands coating the paper...
Cheers & Thanks Annie