If it is something that is already heavily compressed then it is possible for it to be bigger.
Basically it can't make the already compressed data any smaller, but it has to add the data that is necessery to make it a pdf file, hence it's bigger overall...
That's a bit of a simplified explanation, but I hope it makes sense.
However, it doesn't explain it being "Far" bigger. This may be due to some setting you have in the pdf conversion. It might be worth checking Page size, image resolutions, and whether fonts are having to be embedded.