Build a Compiler Bomb
C, (14 + 15) = 29 byte source, 17,179,875,837 (16 GB) byte executable
Thanks to @viraptor for 6 bytes off.
Thanks to @hvd for 2 bytes off and executable size x4.
This defines the main function as a large array and initialises its first element.  This causes GCC to store the entire array in the resulting executable.
Because this array is bigger than 2GB, we need to provide the -mcmodel=medium flag to GCC.  The extra 15 bytes are included in the score, as per the rules.

main[-1u]={1};

Don't expect this code to do anything nice when run.
Compile with:
gcc -mcmodel=medium cbomb.c -o cbomb


It took me a while to get round to testing @hvd's suggestion - and to find a machine with enough juice to handle it. Eventually I found a old non-production RedHat 5.6 VM with 10GB RAM, 12GB swap, and /tmp set to a large local partition.  GCC version is 4.1.2.  Total compile time about 27 minutes.

Due to the CPU and RAM load, I recommend against doing this compile on any remotely production-related machine.