[stunnel-users] zlib compression not working

[webmaster at escoltesiguies.cat]

I have configured both windows client and linux server (samba) with  
zlib compression:

compression = zlib

But I have transferred a txt file of 100Mb in 40 minutes and the same  
file compressed with gzip (22Mb) in 14 minutes.

So I thing compression is not working. Client log tells:
Compression enabled using zlib method
and server log:
Compression enabled using zlib method

How can I verify comression is working? maybe compression is working  
but it is poor?