Authentic Missoni evening gown

I bought this for a wedding and sadly it doesn’t fit it is a EU 40/ size 4 US but fits more like a 2- found in vintage shop for $400 Drop dead gorgeous • Missoni Evening Gown • Purple • Sequin Embellishments • Sleeveless with Square Neckline • Concealed Zip Closure at Side Bust: 28.5" Length: 54" Waist: 26.5" Hip: 32" Clothing Size: S Color: Purple Foreign Size: US4, IT40 Fabric: Not listed, feels like natural blend. Good. Moderate discoloration at front; fabric tag removed; light seam splitting at interior.