Your legs are not suppose to be smaller or the same size as your arms. He needs to take some time off and build up his legs. Once they are brought up i think he will do well
ad has much bigger legs than arms. he lacks outer quad asweep, which when he is alone makes them look underwhelming. on stage next to other cometitors, ad's legs look good. they are thick and have good cuts when hes in shape.
also, you are probably used to seeing videos and pictures of bb'ers on stage. that angle is usually straigh forwar, or bit lowwer than waist looking up. that enhances leg size. AD s videos are all coming from the top, looking dow on his legs. which also does make them look a bit smaller than they are.
could his legs be improved? sure.. like i said, laacks a bit of sweep. but sweep is mainly genetic ad AD cant do much but just hammer away and build up thickness and get shredded.
its not that his lower body is small, its that his shoulders and arms are just so damn big.