ほほえむ (hohoemu) means to smile gently or subtly. In contrast to hohoemu, ニコニコ (nikoniko) describes a broader, more obvious smile, often reflecting genuine pleasure or amusement. You'd choose nikoniko when you want to emphasize the breadth of the smile and the associated positive emotion.

Consider two scenarios:

Scenario 1: A grandmother is watching her grandchild take their first steps.

If the grandmother is softly smiling, with a slight upturn of her lips as she feels warm affection, the situation suits hohoemu. She’s filled with a quiet, internal joy. If, however, her face breaks into a wide, beaming grin as the child wobbles and succeeds, she is nikoniko, projecting her excitement outward.

Scenario 2: A person is trying to solve a complicated puzzle.

If the person is concentrating intently, a slight smile playing on their lips as they get closer to solving it, and finding the process enjoyable, it is hohoemu. If that person suddenly realizes the puzzle's solution, and breaks into a wide, happy grin, delighted by their own cleverness, that is nikoniko.

Let's also contrast it with another similar word: にこり (nikori) which describes a slight, almost imperceptible smile, often fleeting. The 'ri' ending creates the diminutive, and suggests reserve or briefness.

Scenario 3: A person is listening to a friend tell a joke.

If the joke is amusing, and the person gives a small smile, showing they are politely amused, the word nikori is the right choice. If, however, the joke is unexpectedly hilarious, and the person responds by laughing with a big smile, then the situation calls for nikoniko.

Finally, the word 笑う (warau) means to laugh. You'd use warau when describing the action of laughing. You'd choose nikoniko when describing a joyful, smiling state.

Scenario 4: A child receives a special present.

If the child throws their head back and laughs joyfully, you’d describe them warau. If the child shows their happiness through a wide smile, and they're just brimming with pleasure, the word nikoniko is the correct choice.