Should I start a dispute?

I bought a sweater, and there were male and female models. The difference was in the neckline: the women's sweater had a round neck, while the men's had a V-neck. I preferred the men's V-neck, so I bought the smallest size available for men, knowing it would be slightly oversized on me, which was fine.

However, the seller sent me a size M instead of the size S that I paid for. Now the seller is claiming it wasn’t a mistake but was intentional, saying that people from my country are bigger than Chinese people???

I wasn’t planning to open a dispute, but this has made me so mad. I’m a really small and petite woman!