why is the tar.gz file extremely large?

[Wolfgang Engelmann]

I have two graphics next to each other in one figure float, both as .png 
(obtained from pnm files which I got by scanning photographs). The pdf 
file shows the figures with a large gap between them, although I did not 
insert a horizontal space between them.
If I export the lyx file (which has 7.5kB) as a tar.gz, this file has 
53.8 MiB. The size of the graphics are about 180 and 170
kB
What is wrong with the graphics? Do I miss an additional program for 
converting the graphic type?
I could provide the lyx and figures if somebody wants to check it.
Wolfgang