JPG - PNG conversion problem

[Raaijmakers, Vincent (GE Infrastructure)]

Who can explain the huge increase in size when I convert a JPG into a PNG format using PythonMagick:
I read the JPG from a video server and has a resolution of 352x240, size is about 15k
After my PNG conversion and resizing the resolution to 160*120, the size is 64k!!

This is what I do:

##image = jpg data read from a server with a 352x240 resolution

blob = Blob()
blob.data   = str(image)

data_len="jpg length data: %s" % len(blob.data)
logger.info(data_len) ## size is about 15k
     
## convert blob data into a Image
img = Image(blob)
img.depth = 8
img.magick = "PNG"
s = "!%sx%s" % (160, 120)
img.sample(s)

# convert back into data for streaming purposes
blob = Blob()
img.write(blob)

data_len="png length data: %s" % len(blob.data)
logger.info(data_len)  ## size now 64k?!


what is wrong, or what do I forget to do? Is it the direct conversion from JPG to PNG,
do I need to add a step in between to RGB...
Also, tried to play with quality settings on the image, compressionTypes... nothing seem to help.

Needs advice!

thanks in advance,
Vincent