To wear a smaller size, gain weight
Gaining weight, in this case, is referred to as muscle weight. A simple illustration is that of 2 women who both weigh 150 pounds. One lifts the weight, but the other doesn’t. It is more likely that the lady that lifts weight will fit into clothes with a smaller size than the other. With the same logic, a 150-pound woman that lifts weight would fit in the same cloth size of a 140-pound woman who doesn’t work out.
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This is because a pound of fat takes up more space than a pound of muscle. Once muscles replace your fat, you will get smaller which means clothes with a smaller size will fit you.