。
She isn't as tall as he is.
Literal
She [topic-は] him [as-much-as-ほど] back [subject-が] not-high.
The ~ほど~ない pattern is the standard 'not as ... as' construction in Japanese. ほど ('extent, degree') marks the standard, and the negative predicate completes the comparison. Note the 'tall' construction: 背が高い literally 'back is high' — Japanese expresses many physical attributes with ~が adjective rather than English's predicative attribute on the person. The が here marks 背 ('back, height') as the descriptive subject within the topic-comment structure.