。
She is gaining weight.
Literal
She [topic-は] weight [subject-が] is-increasing.
目方がふえつつある — 目方 (めかた, 'weight') is a slightly old-fashioned word for body weight (体重 is more modern). ~つつある marks a gradual change in progress — 'is in the process of increasing.' More literary than ている for ongoing change.