As it turns out the manual is a start and the end of every quest regarding PHP...
I cannot believe that I didn't read this before (more like six years before!!!) or remebered it:
bool imagejpeg ( resource image [, string filename [, int quality]] )
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quality is optional, and ranges from 0 (worst quality, smaller file) to 100 (best quality, biggest file). The default is the default IJG quality value (about 75).
As it turns out I used 100 and when i set this to be 50 I got 4-5 times lighter image! Man, I feel stupid sometimes - but then again I found this all by myself simply by READING the manual... 😃