Is the Cantonese diminutive (粒) the same as the Mandarin (子)?
No.

“鸡粒” is common Chinese, means "diced chicken", “粒” means diced small cubes of meat.

“鸡子” is Cantonese, means the kidney, a viscera of chicken.

Actually different things. But “鸡粒”, “鸡籽”, and “鸡子” may all mean the egg of chicken in loosely.