Are larger characters in males due to sexual selection?

Catalina Sanchez-Quiros and co-workers have published a study on how body traits used in reproduction may have evolved.

Abstract:
Animal body traits are scaled relative to overall body size depending on the evolutionary context. Most naturally selected traits are scaled approximately isometrically (constitute a constant proportion of the body size at different body sizes), whereas those under sexual selection tend to present positive static allometry (be proportionally larger in larger individuals). However, there are body traits that might be influenced by both natural and sexual selection. We studied the courtship behavior of the scorpion Centruroides margaritatus (Gervais 1841) and analyzed the static allometry of several body traits. We hypothesized that those traits that were actively used in courtship and seemed to be sexually dimorphic could be under sexual selection. The main sexually dimorphic traits were body size (female larger) and metasoma length (male longer). Although metasoma length of males had a steeper allometric slope (larger males had longer metasoma) than that of females, the slopes did not differ significantly. All body traits measured showed isometry with body size, except that the pecten presented negative allometry in males. Thus the length of the metasoma of males, thought to be influenced by sexual rather than natural selection, did not present positive allometry as expected. Males used the metasoma actively while courting females.

Reference:
Sanchez-Quiros C, Arevalo E, Barrantes G. Static allometry and sexual size dimorphism in Centruroides margaritatus (Scorpiones: Buthidae). Journal of Arachnology. 2012;40:338-44. [Subscription required for full text, but free access for all after 12 months]