Dramatic improvement in png file sizes

I've been using transcode 1.0 for the last couple of years to extract 
frames from recorded television programs:

transcode -q 0 -o test.$host -y im -F png -x ffmpeg,null -i test.avi 
--frame_interval 1800

The resulting files size have been consistent, in a band around 600kb. 
After installing  the  1.1alpha 7 version on some iMacs, I'm getting far 
smaller files, in a band around 350kb.

When I compare the images, paging between one to the other, I can't see 
a single difference. What's causing this dramatic improvement?

Cheers,
David


517984 Feb 22 20:58 test.1.0-linux0.png
564510 Feb 22 20:58 test.1.0-linux1.png
707892 Feb 22 20:58 test.1.0-linux2.png
409238 Feb 22 20:57 test.1.1-tiger0.png
335458 Feb 22 20:57 test.1.1-tiger1.png
405116 Feb 22 20:57 test.1.1-tiger2.png
409162 Feb 22 20:56 test.1.1-leopard0.png
335470 Feb 22 20:56 test.1.1-leopard1.png
405115 Feb 22 20:56 test.1.1-leopard2.png