The names of the two main characters would seem to each have a double meaning. If we separate the two syllables of Jekyll's name – "je" and "Kyll" – it could mean "I Kill", as "Je" means "I" in French.
Similarly, Hyde's name may appear to suggest other things if we take its homonym "Hide" which could refer to something that is hidden from view or the rough skin of an animal pointing to Hyde's uncivilised and animalistic nature.
In the novel there is a considerable difference in size between Jekyll and Hyde. Hyde is much smaller than Jekyll and this is most notable when his small body is found in the clothes of Jekyll.
As Hyde represents the evil side of Jekyll, this size difference suggests that the author views this side of the human character to be in some way lesser than the good and rational side.
Hyde is also described as being younger and more energetic than Jekyll. This suggests that this side of human nature develops later in life after a period of childhood innocence.
It also suggests that Stevenson felt that this side of human nature was more exhilarating and energetic than the higher more respectable side of human nature. This is also illustrated in the novel where Jekyll describes how much he initially enjoyed being the Hyde persona.