Currently there is only one linkage model, linking for a linux process. Eventually, more may be added to support other hosts or different versions of linux.
This model sets aside a 4Megabyte heap space in the .bss section, so linux will zero-fill it. Also, by placing it in the .bss section, the space will be allocated to the process, no calls to brk(2) required, although it won't consume physical pages until the application touches the memory.
If you want different heap sizes, edit the linux.ld linker file to reflect the desired heap space.