RPM is changing my binary files on build.

I have a build-only RPM that contains two binaries that have been
compiled on RHES4 box. I am actually building the RPM on RH 7.3. I
copy the files into the build root and just execute:

rpm -bb --target noarch <specfile>

what is happening is the file sizes on those two binaries get changed
after RPM is run. Clearly, RPM complains about not having the correct
version of libc, but why would it whack the file? Is there a way to
stop RPM from changing them?

Thanks.