I am assuming you want to know why is is that Hyde is so much smaller, slighter, and younger than Henry Jeckyll?
There are some interpretations. Victorian times were so stuck up and stuffy that many things were considered evil, or polluted, or unfashionable. Repression both mental and physical was rampant and it only hid away the reality of secret vice and inner desires.
Hyde is an allegorical character representing those repressed feelings, and he is an extension of Jeckyll. For this reason the author, Robert Louis Stevenson has to portray him "small, and slight" like someone's shadow would reflect on the ground, and "much younger" is a description that befits immaturity and carelessness. After all, Hyde and Jeckyll both live at each other's shadows.
Mr. Enfield describes in the beginning of the novella as:
"He is not easy to describe. There is something wrong with his appearance; something displeasing, something downright detestable. I never saw a man I so disliked, and yet I scarce know why. He must be deformed somewhere; he gives a strong feeling of deformity, although I couldn't specify the point. He's an extraordinary-looking man, and yet I really can name nothing out of the way. No, sir; I can make no hand of it; I can't describe him. And it's not want of memory; for I declare I can see him this moment." (p.15)
So basically, he is a creepy little man who causes lots of mayhem the way any other dark creature would in folk literature.
When Dr. Jekyll drinks the potion and turns into Mr. Hyde, he says that he feels younger and lighter. At least he feels that way after he goes through the physical and mental pain of the transformation.
He says that the reason for this is that Mr. Hyde is made up only of the evil part of his nature. He says that Dr. Jekyll has spent most of his life trying to be good and doing good things. So naturally his evil side isn't all that big. Because of that, Hyde is smaller and younger than Jekyll. Hyde is younger because the evil part of Jekyll hasn't been used as much and isn't as tired as the good.