Depends what is in the form and what "Save options" you have chosen. IN Designer under File/Form Properties/Save Options you will see three checkboxes. One is to embed the fonts that are being used, one is to make the form Accessible and the 3rd is to generate a log. The first 2 options mentioned will affect the size of the PDF greatly. Only embed the font if absolutely required and turn off accessibility if you do not need it.