Thank you for inquiring. Both reasons given to you for sex by this boy are wrong. The fact that you are in puberty, means that you grew from conception to infancy, childhood and now puberty, all without having sex to help you grow.
Breast size just like growth is also not determined by playing sex. At puberty, a hormone (substance that regulates body processes) called oestrogen, produced by the ovaries stimulates all major parts of your breasts to grow including the fatty tissue which makes the breasts grow bigger.
It is therefore not true that sex would make you grow or make you breasts bigger. Don’t be fooled by the boy who may even be having other intentions.
Your breasts may be smaller than those of your colleagues. This is normal because we all grow at different rates.
It’s not true because the growth of breasts is not determined by playing sex.
It’s not true because girls bodies are not the same. Others develop breasts at 18yrs, so, don’t be worried.
That’s not true because every girl has different hormones. I know of a girl who grew breasts at the age of 16yrs but she is fine and now she is happy with the situation. That boy might just be wanting to lead you into having sex with him. Every body has a different speed of growth. Yours is actually fine.