He/she has something that is a little larger than a pacemaker.
ご質問ありがとうございます。
"He/she has something that is a little larger than a pacemaker."＝「彼・彼女はペースメーカーより少し大きなものを持っています。」
追加：「装着」をこの場合は "implanted"（埋め込んだ）と訳せます↓
"He has a device implanted in him that is a little larger than a pacemaker."
ご参考に。
He had heart surgery and had some kind of medical device that was a little bigger than a pacemaker implanted in his chest.
この場合、次のような言い方ができますよ。
ーHe had heart surgery and had some kind of medical device that was a little bigger than a pacemaker implanted in his chest.
「彼は心臓の手術をして、胸にペースメーカーより少し大きい医療機器を埋め込まれた」
medical device「医療機器」
to be implanted in one's chest「胸に埋め込む」
ご参考まで！
アメリカ合衆国 
カナダ 
