Who said anything about life? A machine-intelligence could operate a tiny craft. And even a biological, think brain in a jar, doesn't need that much space. Astronomers cannot find or disprove the existence of Planet X in the ort cloud, an object at least several times the size of earth. An entire civilization could be out there and we wouldn't notice.

1) life may take different forms, some of which we probably haven't even dreamt about

2) life is not necessary for inter-stellar probes to exist. In fact it probably makes more sense to make these things entirely artificial