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Phrase(s): a drop in the bucket [and] a drop in the ocean
Fig. an insignificant contribution toward solving a large problem. • Jane: We need to stop spending so much. Alan: OK. I’ll buy a cheaper brand of toothpaste. Jane: But that’s just a drop in the bucket. • Many companies donated food and medicine to help the survivors of the earthquake, but it was just a drop in the ocean of what was needed.
McGrawhill's American Idioms And Phrasal Verbs
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A very small quantity, especially one that is too small. For example, These contributions are just a drop in the bucket; the new church wing will cost thousands more. John Wycliffe's followers used this seemingly modern phrase in their translation of the Bible (1382), and it Also appears in the 1611 King James version (Isaiah 40:15): "Behold, the nations are as a drop of a bucket, and are counted as the small dust of the balance."
American Heritage Idioms