merging pdf (repost)

[scarso luigi <scarsoluigi@libero.it>]

Hello, I use this code to merge file1.pdf, file2.pdf, file3.pdf  in
file4.pdf

% format=english
\definepapersize[A3large][width=320mm,height=450mm,offset=0pt]
\setuppapersize[A3,landscape][A3large,landscape]
\setuplayout
    [backspace=0pt,marking=on,
     height=middle,width=middle,location=middle,
     topspace=0pt,
     header=0pt,
     footer=0pt]
\starttext';

\insertpages[file1.pdf][width=0pt]
\insertpages[file2.pdf][width=0pt]
\insertpages[file3.pdf][width=0pt]
\stoptext

where 
file1 is    481 031 bytes
file2 is  1 277 167 bytes and
file3 is     43 574 bytes. I expected ~2 Mbytes for file4.pdf but 
file4 is 10 601 693 bytes, ie ~five times the size expected.
Why ?

Thanks in advance
luigi

PS.
I have a context distribution that is 1 year old, but i can't actually
upgrade it (well, if I must do it, I will do !)