Hi! To increase the size of your chest muscles, you need to do more chest building exercises. make sure you are lifting weights that are heavy enough. You can gauge this by lifting weights heavy enough to fatigue you between 8-12 repetitions. If you are able to do more reps, the weight is to light for you & vice versa. Also make sure you do atleast 3 exercises & 3 sets for every exercise each time you work your chest. Some exercises- push ups, bench flyes, bench press, pec dec etc.