Sebastian, Try adjusting the Compression ratio on 0.33. This can now be done through the FOG GUI under FOG Configuration->FOG Settings->FOG Boot Settings->FOG_PIGZ_COMP.
It has a range of 0 thru 9. A 9 is the highest compression rating and takes the longest time to process and upload as it is compressing data quite a bit. 0 is the worst compression rating, and takes the shortest time to process and upload. In years past, the default was at 3, but it was Hardcoded into the init.gz file. This setting allows you to decide how much compression you would like. It auto set’s at 9 though so I would start here.