You can gain weight. Not sure what you weigh now, but I would not want to be overweight just to have bigger breasts.
If you are under 18 and very thin, I can pretty much guarantee you will have bigger breasts by the time you are 25. If you are very heavy, and you don’t like that your stomach is bigger than breasts, then getting into shape will reduce your breasts and stomach, but your figure overall will improve, and become more hour glass once you get rid of the belly fat.
I never had one change in my body while on the pill. I didn’t get bigger boobs, I didn’t gain weight, I didn’t have shorter periods, I do not recommend birth control to try and get bigger breasts.
What @Pandora recommends about building the muscle is true too, a little build in the muscle under the breasts will help them stand up and give them the appearance of being a little larger, but building the muscle too much will not look soft and feminine, so I prefer not to overwork the muscle too much, but that’s me. You don’t have to worry about it, just exercise the area, and when you are happy you can level out your weight lifting in the area. You want to exercise your pectoralis muscles, if you want to google exercises.
Get those “cookies” or inserts that you can add to a bra, they go usually below and to the side of your breast in the bra. Or, a padded bra for now. Some push up bras make women look very narrow on top, I do not recommend those. Don’t try to push together and get cleavage, just go for a cup size more, it looks more natural. Go to a really good bra shop and let them help you.
How old are you? What is your bra size now? How tall are you? How much do you weigh?