png exported from command line has low density

[shiraz hashim <shiraz linux kernel gmail com>]

Hi,

When I use dia gui to export a diagram in png (pixbuf) format, I get following

$ identify -verbose ex.png | egrep -i "resol|geom|pri"
  Geometry: 306x346+0+0
  Resolution: 72x72
  Print size: 4.25x4.80556

but when I export the same diagram from command line, as following

$ dia -t png -e ex.png ex.dia

I get,

$ identify -verbose ex.png | egrep -i "resol|geom|pri"
 Geometry: 304x344+0+0
  Resolution: 20x20
  Print size: 15.2x17.2

Why the Resolution (density) is low  using command line ?

P.S. I have selected pixbuf png as the preference for png format.

-- 
regards
Shiraz Hashim