How to slim down?

I have had two pregnancies. After the first, ppl all told me I would slim down soon and there’s no need for massage or what. But it’s not really true. I became fatter even though I didn’t gain too much weight. After my second pregnancy, I decided to go for massage which many commented it’s effective in reducing water retention and hence the size. However, it is not true again. I’m now 2 months pp and I become much fatter after this pregnancy. Anyone has any suggestions how to slim down back to pre pregnancy size? I used to be quite skinny before pregnancy, but not anymore. And I felt my pelvic bones had become broader. Help!!!