When using expect to sign RPM's package signing fails to work on larger RPM's

Issue

When trying to use a script (seen below) that includes expect to automatically sign RPM's with a given gpgkey, only RPM's smaller than 20 MB are being signed.

Example script:

$ cat bin/sign-rpm.exp
#!/usr/bin/expect -f
set rpmfile [lindex $argv 0]
spawn rpm --addsign $rpmfile
expect -exact "Enter pass phrase: "
send -- "foo\r"
expect eof

From here ensure the ~/.rpmmacros file has been configured as follows:

$ cat ~/.rpmmacros
%_signature gpg
%_gpg_name foo

The behavior of the script on a large RPM (more than 20 MB) can be seen as:

$ rpm -qpi 50mbexample.noarch.rpm|grep ^Signature
Signature   : (none)
$ bin/sign-rpm.exp 50mbexample.noarch.rpm
spawn rpm --addsign 50mbexample.noarch.rpm
Enter pass phrase:
Pass phrase is good.
$ rpm -qpi 50mbexample.noarch.rpm|grep ^Signature
Signature   : (none)

The package does not get signed, however, an unfinished (and temporary) RPM file can be found under /var/tmp:

# ls -lh /var/tmp/
total 299M
-rw------- 1 bamboo bamboo 299M Sep 25 10:08 rpm-tmp.gTZIEY