Question about ROOT file size

I am running Geant4 using ROOT files for
persistency (to store the G4 hits output).

While running one example - using G4 version 7.1.p01 and
ROOT v4.04.02 - I noticed that the file size does not grow linearly
with the number of events written.

This is the file and event size I get when I run the example
with different number of events:

 1 evt    6 KB -> 6.0 KB/evt

100 evt 552 KB -> 5.5 KB/evt
1000 evt 4.8 MB -> 4.8 KB/evt
5000 evt 30 MB -> 6.0 KB/evt
10000 evt 65 MB -> 6.5 KB/evt
50000 evt 455 MB -> 9.1 KB/evt

In addition, as each hit consists of 4 float quantities, and as each event has 2 to 8 hits
I would expect an average events size of ~80 bytes (5x4x4), so I cannot understand where this 6KB+ per event comes from.

It is clear that I am missing some basic information here
(maybe about the ROOT file structure and compression).
Can anybody give me any suggestion?
Is there any ROOT documentation that I could study apart from
the ROOT manual that I have already considered?

Thank you very much
Marco Serra

We need more information fromn your side to understand your problem.
Could you post somewhere in a public area (AFS, http) a small file
with events?


thanks a lot for your help,
you can find the ROOT files in my public afs area:
(for 10000 events, 5000 events and so on).
thanks Marco