if you have lot of text and the text is getting outlined, then the pdf will be big.
you can switch to a font that can be embedded by scribus 1.4 or switch to 1.5.x for the pdf creation (but if you have lot of text, there is a risk that the text layout will change slightly, since the newer 1.5.x versions have a complete new text engine; just be careful that 1.4 cannot open files that have been saved with 1.5!).