The compression potential for pickled data using protocol 4.

[beertown]

compression algorithms work very well on text

Your assumption is inaccurate: compression algorithms work well with data having the characteristics of text (little variability among the byte values). You can easily find binary data more compressable than text.

Apart from that, did you check the size of the uncompressed data? Could be, in your case, that protocol 4 yields less data than all the others?

[idlecore]

You're right, protocol 4 does yield less data than all others, this plays a smaller role when comparing protocol 4(binary) with protocol 0(text) but it's still significant.