。
She's been losing weight.
Literal
She [topic-は] body-weight [subject-が] decreasing-came.
減ってきた combines 減る ('decrease') with the auxiliary ~てくる. ~てくる marks a change-of-state developing toward the present — 'has been increasing/decreasing/coming about up to now.' Pairs with ~ていく for the future-oriented counterpart ('will continue to'). Here 減ってきた captures the gradual progress of weight loss as observed up to the moment of speaking.