Dog has thin legs then his overall body

Hello,
   
Sir/Madam,
   
My name is 'Bhavin' and i have dog pet the breed is 'St.Bernard', he is 14 months in age , he is in good health, good height, but i am worried about his legs as they are not 'thick and muscular' as i see in pictures when i google, they are bit thin as his body is in good shape and health but legs are thin when we see them specially the 'rear legs' because of that he is not able to run for long time, please help what i should do to make his legs overall 'thick and muscular'.
   
Thank you,
   
Bhavin Patel