Thanks for the advise, Peter.
But modifying the JSONIFY_MAX_FILESIZE envvar did not help. (When I call get_item for the pdf and image files directly in the Plone 3 site, I get them in full size. As transmogrifier uses this procedure, this part of the thing seems to work fine).
T.Kim, can you remember any link to any special treatments of pre-blob storage sites?