He makes a bigger splash because he weighs more than the other kids.
He's bigger than the other kids, so the water shoots up higher when he jumps in the pool.
ーHe makes a bigger splash because he weighs more than the other kids.
「彼は他の子より重いから、大きな水しぶきをあげる」
to make a bigger splash で「大きい水しぶきをあげる」
to weigh more than ... で「…より(体重が)重い」
ーHe's bigger than the other kids, so the water shoots up higher when he jumps in the pool.
「彼は他の子より大きいから、彼がプールに飛び込むと水しぶきがより高く上がる」
the water shoots up higher で「水しぶきがより高く上がる」を表現することもできます。
ご参考まで!