if in the version you were subsetting and in the second included the otf font, this would probably explain the increase in the size of the pdf file.
but since you're using 1.4.8, you probably cannot include otf fonts at all.
anyway, the pdf is composed by the glyphs placed on the page, copies of the images and the font files.
the sum of those elements makes the pdf size.
if you did not add text, did not change anything in the fonts used (even for things you have deleted afterwards), nor touched at the images (and at the way they are included in the pdf), you might to create a new file that is similar to what you have (but only contain things you can share), make copies each time you save it and create the pdf and if you can reproduce again the issue, you can then share the files so that we can look at them!
scribus is known for creating rather big pdfs, but i've never heard of anyone who experienced a file growth that had no reason...