Do cells shrink or disappear when losing weight?

[Le nouveau clavier de Todd]

Does weight loss in humans mean that the person has fewer fat cells? Or, does the number of cells stay the same, but the size (and mass) of the cells are reduced?

Similarly, if one gains muscle mass, are new cells created, or are existing cells getting larger?

I assume that the purpose of radiation and chemotherapy is to destroy cancer cells, not just make the cells smaller. Have to admit, I don't know.

thanks, Todd's seeking an answer key board