It looks like he might fall backwards because of the weight of his backpack.
ーIt looks like he might fall backwards because of the weight of his backpack.
「ランドセルの重みで後ろにひっくり返りそう」
to fall backwards で「後ろにひっくり返る」
the weight of his backpack で「ランドセルの重み」
ーHe's so small and his backpack is so heavy, he looks like he might fall straight backwards.
「彼はすごく小さくて、ランドセルがすごく重いから、彼は後ろにひっくり返りそう」
to fall straight backwards でも「後ろにひっくり返る」を表現できます。
ご参考まで！
カナダ 
