Negative compression gains can result when the original packets are not compressible. The post compression IPCOMP packet ends up being larger than the pre-compressed packet. In order for positive compression gains to take place, the original packet must be compressed by a greater number of bytes than are added by the additional IPCOMP packet header.
The compression show output is a snapshot showing the compression gains for traffic types over the last eight packets compressed. For a more realistic view of compression gains, use the compression reports for those classes to measure compression gains over a longer period of time.
If class compression reports consistently show low or negative compression gains, compression for a class can be disabled using the command:
class compress <tclass> off