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。