Dictionary / Vocabulary / 大は小を兼ねる
だいはしょうをかねる
the greater embraces the less; better too big than too small
ちょっと大きいけど、この封筒でいいや。大は小を兼ねるって言うし。
It might be a bit big but this envelope will do just fine. It's better to be too big than too small.