Though uLisp has tail-call optimization, if the C compiler doesn't, then the garbage collector needs stack in proportion to list length:

  void markobject (object *obj) {
    if (obj == NULL) return;
  
    object* arg = car(obj);
    if (marked(obj)) return;

    int type = obj->type;
    mark(obj);
  
    if (type != SYMBOL && type != NUMBER) { // cons
      markobject(arg);
      markobject(cdr(obj));   // <--- TAIL CALL!
    }
  }

It's simple enough to obj = cdr(obj) and wrap a loop around this.