Proverb meaning "the big can serve as the small" — it's better to choose something slightly larger because it can fulfill the role of a smaller item.
服は大は小を兼ねるから、少し大きめのサイズを買った。
Clothes can be worn even if they're a bit big, so I bought a slightly larger size.
保存容器は大は小を兼ねるという考えで、余裕のあるサイズを選んだ。
I chose a roomy size for the storage container, thinking 'bigger can serve as smaller.'
会場は広めに取った。大は小を兼ねるから安心だ。
We reserved a larger venue. It's reassuring because bigger can serve as smaller.