When I build Heavy myself, it's bigger and not exactly the same as the Heavy.bin available for download

The version of Heavy.bin that we provide for download is always built with Rowley CrossWorks.  CrossWorks uses a slightly different version of GCC, with a proprietary C runtime library.  This usually results in smaller build sizes than some of the GnuArm toolchain builds, largely because newlib is not quite as optimized as it might be.  But because CrossWorks does indeed build with GCC, the functionality should be very similar (effectively the same in most cases).