pdf file size

[sefer-li]

I compose mixed graphics-text files in word and convert them to pdf for publishing on my web site. In order to permit them to load quickly I try to make the pdf files as small as possible.
usually the converted files are aproximately the same bit size as the .doc files.
but today I converted a word file of ~450 kb and received pdf of ~3mb.
the document is mixed b/w pictures and text. the total original size of pictures doesn't exceed 400 kb and text ~ 40 kb.
what can be the reason. what should be the attributes of word files in order to convert them into smaller pdf.
please advise
thanks