The word 'greedy' is a stigmatic one, and carries cruel stereotypes. Furthermore, it pathologises being hungry, which isn't good! If a child is showing signs of overeating/eating when not even hungry, the solution isn't to shame/embarrass/humiliate them, all of which calling them 'greedy' will do. The solution is to gently and kindly figure out what's going on - why they're overeating, if something is happening which is causing this, etc.
Additionally, lots of kids are chubby. This isn't necessarily indicative that anything is massively wrong! Bodies are differently shaped, plus kids are growing - the body often puts on weight before going through a growth spurt. For example, upon reaching puberty, a lot of girls put on LOTS of weight, which isn't a bad thing - it's a sign they're developing properly!
If you're concerned about a child's weight, the solution is to give them healthy food, teach them about getting more in tune with hunger and fullness cues in the body, and getting them into an exercise program they enjoy (eg it could be any sport - if they don't like team sport, they might like swimming with you!). The focus should absolutely NOT be on weight - they don't need to feel body shame - it should be on getting healthy, feeling good and having fun together!
If you're concerned that the child has an eating disorder (e.g. binge eating), the solution is not to shame them by calling them names, but to get them professional help.