Don’t buy a gown more than one or two sizes larger than the size you actually wear.
Larger gowns are cut proportionally larger all over the gown. The neckline is wider, the bust line will be off center, and shoulders will hang off.
You can’t take in all the extra fabric through the sides.
The dress was constructed in sections. Each piece was cut to a larger size and then put together to be the size it is.
The dress will look distorted unless you take in each seam, in even amounts, until you have taken in the amount you need to be your size.
This would be like re-making the dress. And would be a very expensive alteration!