。
She was heavyset, but she danced beautifully.
Literal
She [topic-は] body-weight [subject-が] was-heavy [but-が] skillfully danced.
重たい is a casual variant of 重い ('heavy'); the ~たい ending here is not the desiderative want-to-do form, but a parallel adjective formation found in a few words (重たい / 眠たい / 煙たい). The clause-final が is the contrastive 'but' — the same が that marks subjects, used between clauses to soften the transition. Here it sets up a contrast between the speaker's expectation (heavy = clumsy) and reality (she danced well).