About linux : sigaction-argument-ucontextt

I set the SA_SIGINFO flags to handle the signal.When I use gdb to check the argument.I found info point to 0x7fffffffd2b0 and context point to 0x7fffffffd180. I kown the true type of argument context is ucontext_t.Strangely, the space 0x7fffffffd2b0 - 0x7fffffffd180 == 0x130 is much less then sizeof(ucontext_t) == 0x3c8.So the memory which argument context points to can not be a complete ucontext_t structure. Why this happened? Am I miss something?

void recvSignal(int sig, siginfo_t* info, void* context)
{
    int a = 0x12345678;
    printf("received signal %d %d!\n", sig, a);
}