I would say casque size is a tradeoff as there are undoubtably costs and benefits to increased casque size. It would be energetically costly to produce a large one, for instance. I would also say, however, that if the casque was under selective pressure as an adaptation for something like drinking, that this pressure would at least be exerted equally on both sexes of a single species and you wouldn't expect to see such a huge dimorphism in the trait between males and females. The fact that there are such strong male-female differences in C. calyptratus suggests to me that it has another form of selective pressure, such as sexual selection and/or species recognition, exerting a large amount of influence.
Some work by Rand (1961) examined the function of ornamentation in chameleons. In comparing the anatomy of sympatric species in East Africa found that 13 of 14 species examined differed from a sympatric species by only a single character, suggesting that ornamentation is used frequently for species recognition purposes through the process of character displacement. Casque size also relates closely to bite force in many species (Stuart-Fox et al. 2006; Karsten et al. 2009; Measey et al. 2009; da Silva et al. 2014; Ligon & McGraw 2018). Correlations with prey type and sex in different species show that this may be driven by both natural selection on prey hardness and on sexual selection (male-male combat), depending on the species.
Anyway, I have no doubt that the casque can be secondarily used for a variety of things, but I don't think there is strong support for drinking being the primary driver of its development and evolution.
Chris
