question about Red Hat update and size of rpms

[arnau mir uib es]

Hello.
I have updated from Red Hat 7.0 to Red Hat 7.3 and the size of the hard 
disc increased  in about 700Mb.

So, I try to find all the size of the all installed rpm files with:

rpm -qa --queryformat '%{name} %{size}\n' >  files_rpms

Next, I import the file files_rpms in a worksheet and I added the sizes 
of all rpm files.

The result was 1.61 Gb. but If I do
df
the result was 3.1 Gb.


My question is: where are the remaining 1.5 Gb.? or where does the 
instalation save their old rpm files?

Or, how can I find the rpm files or programs that are not useful?

Thanks in advance,

Arnau.