a drop in the ocean; a drop in the bucket
Idiomatic expression meaning something is so small or insignificant compared to the whole that it makes almost no difference. Often used to describe a contribution, effort, or amount that is negligible in the grand scheme of things.
I donated, but it's just a drop in the ocean.
This level of saving is a drop in the bucket.
Similar meaning: 'a single hair from nine oxen' — also expresses insignificance within a vast whole. 大海一滴 emphasizes the ocean/water imagery, while 九牛一毛 uses the image of a tiny hair among many oxen.
A yojijukugo (four-character idiom) of Chinese origin, literally 'one drop in the great ocean'. The exact historical derivation is uncertain, but the phrase is conventionally associated with the idea of extreme insignificance.