How big is the file? Can you attach it here? It is hard to tell what the situation is without seeing the file.
Remember that the PDF is font-based while the png is pixel based, so it may well be larger. Preview usually does a good job of creating small files, but if it has added an icon resource, that can make the file much larger (on the order of 100KB at times), so it might pay to remove that if it has one. I can tell you if you I can see the file.
PREP 2013 Question Authoring - Archived
pspicture (LaTeX packages)
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