Hi Nitty Gritty,
Your breast implants are not the cause of your weight gain, or your growth in your breasts over the last 18 years. However, your weigh gain, along with your own genetic predisposition, is responsible for your breast growth over these years.
Like most other americans, you are gaining about a pound a year. Unfortunately, it is very common for people to feel like they are doing everything right with their diet and exercise program, yet they cannot get the results they were hoping for. We seem to get innundated with information about diet and exercise from everywhere we look, but rarely do people get the right information. A quick word about getting lean: it's 85% diet related. Exercise is important, but just as a backdrop for a life long healthy diet which can get you to the weight you want to be. The evidence is growing regarding what the diet should be. The New England Journal of Medicine just published an article last week about the superiority of a high protein/low glycemic diet. Call it what you will: Atkins, Susan Summers, South Beach, or Zone. They are all variations on the same theme, and you don't need to follow them to the letter.
As for your breasts, your options depend on their appearanc and the level of your desire to change them. The simplest thing to do would be to remove them and replace them with smaller implants or not to replace them at all. Depending on their appearance, a breast lift might be indicated as well. If you are interested into looking into your options, you should have a consultation with a board certified plastic surgeon that specializes in breast revision surgery.
I wish the best of luck to you!