Although sexual size dimorphism is a widely observed phenomenon in nature, the selective forces that led to it are still controversial. Here we study sexual dimorphism in the static allometry of the legs of a large ground spider, Grammostola rosea (Walckenaer, 1837). We found that this species has a moderate sexual size dimorphism and males have longer legs relative to body size than females, similar to other ground spiders. We propose that male mate searching behavior may be a relevant factor in the genesis of this phenomenon. The longer extremities in males with respect to mass than in females would lead to an optimization of the costs associated with locomotion, because males have smaller masses and longer legs than the females both in absolute terms and relative to body mass.