Oversized is usually used when talking about clothes; when a sweatshirt, a jacket or a pair of sneakers are oversized, it means that even if they're your size, they seem much bigger, swallen or blasted. If you wear a jacket larger than your size, it's not oversized, it's just not your size
"Daaamn Alex, where did you get that jecket from?"
"This? You like it don't you? It is oversized, and makes me seem much bigger than I am"
"Yaeh cool, but where did you get this from?"
"I ain't tell ya, I know nigga, you'd copy it, you bastard"