Well to be fair to MT, the dress fit her body size, what it didn't fit was the occassion (or her age and position):
Look at the side seams : they go flawlessly from skirt to bodice, showing that the fabric wasn't pulled.
What the seamstress did, was putting in a modesty piece, because the original version came with an incredibly low back (practically backless) and neither her age, nor the occassion allowed MT to wear such a daring cut.
IMO this was one instant in which a bespoke piece should have been ordered: with a back design that suited MT.
Seems like she should have chosen a different dress.
What is the circled area on the Meghan picture? It's dark so I can't really tell.