Something terrible must be done to png pictures, either in ConTeXt or in luatex.
I have the following minimal sample code and picture. I include the png for those who want to check.

% Test png addition.
Place png figure.\par
\placefigure{Test figure}{\externalfigure[test.png]}

Without the png-picture it makes an 8KB pdf.
But with the png-picture the pdf swells to 1.8MB.
And that for a png-picture of only 48KB.
Moreover making a zip of all this stuff brought the size back to 110KB.
In the log: 
ConTeXt  ver: 2011.11.04 14:15 MKIV  fmt: 2011.11.10  int: english/english
mkiv lua stats  > graphics processing time  - 0.001 seconds including tex, 1 processed images

What is done with the png and can it be remedied? 
This swelling up of small picture to such enormous proportions makes the inclusion of png-picture nearly impossible, at least impractable.

Hans van der Meer