Obesity doesn't happen overnight. It develops gradually over time, as a result of poor diet and lifestyle choices, such as:
eating large amounts of processed or fast food that is high in fat and sugar
drinking too much alcohol – alcohol contains a lot of calories, and people who drink heavily are often overweight
eating out a lot – you may be tempted to also have a starter or dessert in a restaurant, and the food can be higher in fat and sugar
eating larger portions than you need – you may be encouraged to eat too much if your friends or relatives are also eating large portions
drinking too many sugary drinks – including soft drinks and fruit juice
comfort eating – if you have low self-esteem or feel depressed, you may eat to make yourself feel better
Unhealthy eating habits tend to run in families. You may learn bad eating habits from your parents when you're young and continue them into adulthood. 
Lack of physical activity is another important factor related to obesity. Many people have jobs that involve sitting at a desk for most of the day. They also rely on their cars, rather than walking or cycling.
In some cases, underlying medical conditions may contribute to weight gain. These include:
an underactive thyroid gland (hypothyroidism) – where your thyroid gland doesn't produce enough hormones
Cushing's syndrome – a rare disorder that causes the over-production of steroid hormones

Mariah Carey 
Her weight has fluctuated over the past years. It is believed that she is in love with eating pizza and sleeping till midday. There is also a hand of having given birth twice in her chubby looks.