She's overweight and wants to lose some.

Literal

She [topic-は] obese [so-なので] weight [object-を] want-to-reduce [quotative-と] is-thinking.

肥満 ('obesity') is a medical Sino-Japanese term. 体重を減らしたいと思っている — the chain of ~たい + と思う + ている layers desire, thought, and ongoing state.