She's so gaunt, her thighs are much thinner than mine.
She's nothing but skin and bones, her legs are way skinnier than mine.
gaunt を使って言う場合は、次のように言えます。
ーShe's so gaunt, her thighs are much thinner than mine.
「彼女は痩せこけていて、太ももが私のよりずっと細い」
または skin and bones を使っても表現できますよ。
ーShe's nothing but skin and bones, her legs are way skinnier than mine.
「彼女は骨と皮だけで、彼女の足は私のよりはるかに細い」
ご参考まで！
She's so gaunt, that her thighs are skinnier than mine.
She's so skinny, that her thighs are thinner compared to mine.
「彼女は痩せこけていて太股が自分よりはるかに細い」は英語で「She's so gaunt, that her thighs are skinnier than mine.」と言います。英語は何かを比較する時は「than」や「compared to」を使います。
彼女は痩せこけていて太股が自分よりはるかに細い。
She's so skinny, that her thighs are thinner compared to mine.
カナダ 
