NFA Test Case

Dear Sir/Madam,
We are testing the NFA software in our customer site. and there are some problem during testing. We ftp a larget file from ftp server to ftp client. Please help us explain why ftp file size is 5.36GB and NFA just show volume is 2.47GB.
Thanks

Frank

Test Case Study:

Object:
Testing to transfer a large file from ftp server to ftp client
Reviewing the result for Netflow Analyzer for Volume, Package

ftp server description:
OS: IRIX
IP Address: 10.166.4.7

ftp client description:
OS: Windows2000
IP Address: 10.166.3.66

ftp file information
name: test.data
size: 5369709120 bytes

network enviornment:
Cisco 6513 IOS12.2
Netflow V5 enable

NetFlow Analyzer Information:
OS: Linux ES4
Version: 4.0.2
Configuration:
Setting up two IP group for monitoring
One: 10.167.3.66
Another: 10.166.4.7

Testing Result Recording as follow:
NetFlow Analyzer Software:
10.167.3.66 IP Group
Volume: In- 2442.17MB
Out- 170.72MB
Package:In- 7355632
Out- 3706879

10.166.4.7 IP Group
Volume: In- 175.93MB
Out- 2468.61MB
Package: In- 3761949
Out- 7408140

Windows Recording Result As follow:
File Size: 5369,709,120 bytes
Network Interface Package Number
Sender Package number: 1853305
Receiver Package number: 3677488

Please help me explain as follow challenge:
ftp physical file size: 5369709120 bytes
and netflow analyzer just show volume size: 2442.17MB