It has been said over and over again: The size of Scribus PDF does not mainly depend on the graphics, but on how Scribus places text on page.
Scribus places each single glyph on the page separately, most other applications place glyphs one line at a time, but that does not give the accuracy Scribus can give.
So, for example OO Writer would do something like:
Set cursor at x,y
Write text "Hello World!"
While Scribus will:
Set cursor at x1,y
Write text "H"
Set cursor at x2,y
Write text "e"
Set cursor at x3,y
Write text "l"
Set cursor at x4,y
Write text "l"
Set cursor at x5,y
Write text "o"
Set cursor at x6,y
Write text " "
Set cursor at x7,y
Write text "w"
Set cursor at x8,y
Write text "o"
Set cursor at x9,y
Write text "r"
Set cursor at x10,y
Write text "l"
Set cursor at x11,y
Write text "d"
Set cursor at x12,y
Write text "!"
All those cursor relocations take up space in the PDF.
The reason Scribus does like this is because it is targeting at printed materials, where you want that accuracy.