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Not necessarily. Size is a very variable feature of spiders (and often other species). Because spiders have no clearly defined growth stages in the way insects do, the only certain way of knowing if a spider is adult is by whether or not they have the complex adult male palp or female epigyne present. With Segestria females this is difficult, because this is a genus where there is no clearly complex external epigyne present, just an slit-like opening underneath. This is difficult to see even under a low power microscope, and certainly can't be seen by sight or from a photograph. Large size may suggest an adult, but there is no certainty.