Will a bearded dragons growth scale be effected by how big the tank its in ?

ive had my bearded dragon for over a year and a couple weeks now and im kind of concerned about how big he is. hes about 17 inches long but hes not very wide and his head isnt very big. i have him in a 55 long gallon tank and hes always had proper nutrition. im just wondering how if it is normal for them to take a while to get broader or not. oh and by the way do you think its ok to feed him lizards from outside or no? thanks :)