You can encourage young children by calling them a "big boy" or "big girl" when they do something impressive for their age like:
- sitting upright without help for the first time
- using the toilet correctly
- cleaning up their toys
You can use this phrase with a child until about age 4. After that, most children feel like they're already a "big boy" or "big girl", so they don't want to be called that.
Calling an adult a "big boy" or "big girl" has a completely different meaning. It means that they're fat.