It is true that hamsters can sometimes bar chew if a cage is a bit small for their needs however hamsters can bar chew for other reasons which could include frustration because something like a wheel or a water bottle isn't working properly, being able to smell something outside the cage, attention seeking and then there are others who do it just because they can and it has become a habit.
The first thing I'd do is check everything inside the cage works and is accessible and then go through a process of elimination other possible causes. Check your hamster's teeth and that they have something to gnaw on, check the area around the cage for possible things they might be interested in. If you can't find any obvious cause then it may just have become a habit.
There are some tips on managing bar chewers in this post:
Managing Bar Chewers