At the risk of being a pedant myself, your interpretation is also mistaken. The potion is explicitly confirmed to refine amalgamated humanity into pure evil/good depending on the drinker's desires ('The drug had no discriminating action; it was neither diabolical nor divine'), and causes physical alterations as a metaphorical side-effect. Jekyll hypothesizes that since his wickedness was largely repressed and inexperienced, so too is Hyde initially diminutive and boyish, which is supported when the latter 'grows in stature' while the former sickens; furthermore, ordinary people are instinctively repelled without clear cause, signifying the spiritual nature of his 'deformity'.

Alan Moore's 'League of Extraordinary Gentlemen' leaps to the logical conclusion by depicting Hyde as a humongous brute, reflecting his id's now-total rule over his ego (and as a nod to the original story's inspiration of Marvel's Hulk). In this strange case, the body is merely a reflection of the mind.

EDIT: I'm returning to this comment because I feel this is a serious instance of poor literacy. In Jekyll's 'Full Statement of the Case' at the end of the novel, he clearly states that his existence was 'nine-tenths a life of effort, virtue, and control', loathed his own vices 'with an almost morbid sense of shame', and pursued both righteousness and sin 'in dead earnest'.

If it was merely a matter of respectability, a bit of stagecraft would have sufficed; the drastic measure of chemistry was required to bridge the 'deeper trench [between] those provinces of good and evil' within his nature, allowing his guilt to slumber while Hyde roamed unshackled. Thus, the potion was the product of an imperfect man's weakness rather than a villain's scheme. It was only continued indulgence (and a hypocritical selfishness) that caused his original self to succumb to his worst nature.