7zip zip file size growth

Ok i have 140,000+ exchange log files that i need to get rid of, but they are not to be deleted. so instead of keeping 140GB on my server, i thought i would zip them. since it is all text and should zip quite nicely. so i wrote a script to seperate all 140,000+ logs into groups of 500 to be zipped. This is my result of the first 7 groups.

1) 1.754MB     <----What i expected
2) 129.455MB <----A little large, and as you can see it gets bigger
3) 181.573MB
4) 236.767MB
5) 249.138MB
6) 73.349MB   <----Now small again?...
7) 209.941MB

So my question to you all is, am i doing something wrong? nothing should be running on the pc in the background but i thought that it would just take longer to zip the files, not lower the quality.

This is what i am zipping with and the file specs
7za.exe                  <----7zip command line exe
Ultra                       <----7zip highest compression
EachFile is 1024KB <----All are the same size.

Any Ideas? i would like them to be 1MB if possible like the 1st group. :)

Thanks!
FnGizzardBrainAsked:
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Dr. KlahnPrincipal Software EngineerCommented:
The fact that the log files are the same size is misleading in this case.  They won't all compress to a standard size, even though the source files are the same size.

A log file that was used very little will compress very well, because most of the contents are unused.  A log file filled with dissimilar transactions is not going to compress well, because there is little redundancy, while a log file filled with similar transactions and similar timestamps will compress well due to considerable redundancy.  There may be considerable variation in compression efficiency.

I would, however, look very carefully at that first one that compressed 500 1 MB filed down to 1.7 MB.  The only way I can see that happening is if the files were filled with zeroes.
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FnGizzardBrainAuthor Commented:
I see, i had a feeling it would be the contents not just the size. ok thank you for your input i will explore this and comment again tomorrow. Is there a way to compress that type of file more so than what i am getting? Any other program or type of compression i should look into?

Thank you
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Dr. KlahnPrincipal Software EngineerCommented:
You could look into bzip2, but I'm not sure whethere it's going to provide significantly better results.  Still, a byte saved is a byte earned.
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FnGizzardBrainAuthor Commented:
I actually had some free time, i opened up several files from the first group and the second group to compare. And you are right, the first group was not blank however it is filled up with the same characters over and over again. Unlike the second that is a random assortment of characters and symbols. So that answered my question now i have a new one.

Is there a compression program that will do what i want?
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FnGizzardBrainAuthor Commented:
Ok sounds good, Thank you for your help Drklahn!
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