There are a number of reasons why someone may have an oversized mons pubis.
If you gain weight, excess fat tends to accumulate in the mons pubis, causing it to bulge and enlarge and stretch the covering skin. When this weight is lost, without enough elasticity, the excess skin doesn’t return to its normal size and sags. Sometimes this tendency to accumulate fat in the area is inherited from your parents.
Women who have previously undergone a C-section may find themselves left with a bulging mons pubis. This is because after a C-section the skin around the scar may stay right against the tissues, but this is not the same for the skin below it.
Oestrogen can cause increased fat deposition in your mons pubis, hence the mons pubis is naturally more prominent in women.