Try to increase 'memory_limit' value in php.ini file. By default Sitebuilder uses it's own PHP installation and php.ini file for it located here: /opt/php52/etc/php.ini.
To make sure that appropriate php.ini will be used you can run following commands:
# sb_config --help|grep php
--phpcli_path PHP CLI executable [/opt/php52/bin/php5]
--phpcgi_path PHP CGI executable [/opt/php52/cgi-bin/php5]
# /opt/php52/bin/php5 -i|grep php.ini
Configuration File (php.ini) Path => /opt/php52/etc/php.ini
Try to increase value from 32M to, for example, 256M and try backup procedure one more time. It should help.