The answer to this question comes in Chapter 2 of this great novel, entitled "Murdering the Innocents," which describes the Gradgrind philosophy and process of education and its emphasis on fact. What is important to notice is how in a sense both Sissy and Bitzer function as symbols in the novel. Sissy represents the power of creativity, emotions and the imagination, which prevail even under Gradgrind's best efforts to stunt her by focussing on facts alone, whereas Bitzer is a true success story in the novel - he has been formed by Gradgrinds utilitarian philosophy and is completely detached from human warmth, kindness and emotion, as Gradgrind finds out to his cost at the end of the novel.
In this chapter, then, Sissy and Bitzer are compared and contrasted in their schoolroom, Sissy being "irradiated" by a ray of sunlight:
But, whereas the girl was so dark-eyed and dark-haired, that she seemed to receive a deeper and more lustrous colour from the sun when it shone upon her, the boy was so light-eyed and light-haired that the self-same rays appeared to draw out of him what little colour he ever possessed. His cold eyes would hardly have been eyes, but for the short ends of his lashes which, by bringing them into immediate contrast with something paler than themselves, expressed their form. His short-cropped hair might have been a mere continuation of the sandy freckles on his forehead and face. His skin was so unwholesomely deficient in the natural tinge, that he looked as though, if he were cut, he would bleed white.
Symbolically then, Sissy, when exposed to nature, becomes more beautiful, whereas Bitzer, when exposed to the same force, has what little life and vitality there remains in him bleached out of him even further, representing their different functions in the novel.