A comparison based on the debian source packages. Performed the summer 2002.

Packer	Total size	Compression	Improvement	Loss from best	Time (s)
------	----------	-----------	-----------	--------------	--------
tar	25764.5Mb	0		-2.42284	0.786226	0
gzip	7527.25Mb	0.707845	0		0.268287	41010.3
zzip	5987.13Mb	0.767621	0.204605	0.0800635	441934
szip	6501.28Mb	0.747666	0.1363		0.152816	37523.5
bzip2	6479.37Mb	0.748516	0.139211	0.149951	43998.6
PPMd2	8631.81Mb	0.664973	-0.146742	0.36192		36657.1
PPMd3	7199.65Mb	0.72056		0.0435215	0.234993	39515.3
PPMd4	6549.61Mb	0.74579		0.129879	0.159067	41225.7
PPMd5	6223.04Mb	0.758465	0.173265	0.114937	43005
PPMd6	6027.49Mb	0.766055	0.199245	0.0862218	44711.3
PPMd7	5892.44Mb	0.771297	0.217185	0.0652795	46276.5
PPMd8	5796.15Mb	0.775034	0.229978	0.0497504	47621.4
PPMd9	5731.01Mb	0.777562	0.238631	0.0389513	48880.5
PPMd10	5688.98Mb	0.779193	0.244215	0.0318506	50777.1
PPMd11	5661.65Mb	0.780254	0.247846	0.0271773	50990.6
PPMd12	5638.57Mb	0.78115		0.250912	0.0231944	51868.9
PPMd13	5625.2Mb	0.781669	0.252688	0.0208738	52697.2
PPMd14	5613.63Mb	0.782118	0.254225	0.0188553	53441
PPMd15	5609.34Mb	0.782285	0.254795	0.0181046	54141.2
PPMd16	5605.05Mb	0.782451	0.255366	0.0173525	54776.9

Legend
~~~~~~

bzip2		= bzip2 -9
PPMdx		= ppmd -o x -m220
Compression	= Relative compression.
Improvement	= Relative improvement from gzip compression.
Loss from best	= Relative size difference compared to using different
		  compressors for each package and using the individually
		  best compressor for each package.
Time (s)	= Total compression time, in seconds.

For more info about the internals of PPMd, see Dmitry Shkarin's article:

<http://DataCompression.info/Miscellaneous/PPMII_DCC02.pdf>
