If you have a LiveCycle Server software installed, then by rendering the PDF will reduce the size drastically. But other wise, you need to consider the amount of fields you are using on the form and the fonts used in the form..
If you use MyriadPro, it will take less space.
Also under File -> Form Properties, under Save Options tab, remove the Embed Fonts checkbox, this may reduce size of the form.
Thanks
Srini