Joe Lloyd from LA Fitness replies: There are a number of reasons why one side of your chest is smaller. It could be genetic, you may not be training correctly or you might be very left-sided, particularly when playing sports such as squash or football.
If it's genetic there's not a great deal you can do about it, but there are a few things you could try if your shape is simply due to bad posture.
Start by examining your posture while training. Stand in front of a mirror to make sure your body, particularly the hips and lower body, is balanced and that you are not leaning too far to your left. You should also get an instructor to look at your gait.
A dumb-bell press will help build up muscles, as this form of training isolates individual muscles. Although your right chest needs building up, don't concentrate on that side alone as you need to maintain a balance - you don't want to reverse the problem.
It could also be that the muscles on the right are too tight and, therefore, are shorter than those on the left. If this is the case you may benefit from a sports massage that would help relieve tension in those muscles and lengthen them. Chest stretching would also help to lengthen the muscles.