I don’t think that a poor diet is directly related to this decrease in size but it may have a secondary effect as to the look of your body in general. Proper nutrition is important not only for health but for the way we look as well. As our health increases, the look of our body reflects this health. Just as a plant will grow to look more vibrant given the proper nutrients and soil to grow from. Everything from our skin, hair and eyes will appear more vibrant given proper exercise, nutrition and hydration.
Training your chest will definitely have an effect as far toning the general area but will do little as far as breast size or lift is concerned. Training the chest means strengthening the pectoralis major and pectoralis minor muscle groups. The breast tissue actually sits on top of these muscle groups but is not part of these muscles per se. So although you will be toning the underlying muscle you cannot tone the breast tissue itself as it contains no muscle. This will likely improve the overall look of the area but will do little as far as the breasts themselves are concerned.
I realize this isn’t the best news but it’s a truth you have to come to terms with. Unfortunately there are some attributes, surgery aside, that we have to live with. If you are interested in some toning exercises for your chest and arms you can find many to choose from as well as how to perform them in the exercise guides area.
Your best bet is to embrace your smaller cup size and take it as a marker of your improved fitness and health. There are also many fashion styles that accentuate a smaller bust as well as ways to add a bit of makeup to enhance your cleavage.