1. Water retention
2. Bone weight
3. Muscle weight
4. Excess fat
Belly muscles will increase only if you workout and tone the muscles in the abdomen. If your weight gain is through eating alone then you are more likely to store fat in the belly area. Belly muscles do not bulk up on their own. Muscle weight will be marginal as compared to weight gained because of stored fat.
"A love affair with knowledge will never end in heartbreak" - Michael Garrett Marino
Well done is better than well said
'Idhuvum Kadandhu Pogum "
Even this challenging situation would ease