Well, in this setting the explanation kinda makes sense, but it has to be seen in a certain way: if channeling magic puts a strain in your body, the stronger the body, the more magic you can cast and the stronger the spells you can use. Beast races don't have that much potential for magic or not to be that confident in their use.
And for using little clothing... magic beings are not stupid and it's shown, so even if they want to show off, those who do so must be confident on their strength and/or magic to cover the lack of defense for that. Should I be worried about an arrow to my unprotected knee if I can turn my skin, tendons and bones into flexible steel in microseconds? Or roast the archer even before he draws the bow?