https://timesofindia.indiatimes.com/life-style/health-fitness/photo-stories/heres-why-toilet-flush-has-one-large-and-one-small-button/photostory/64540715.cms

IT IS FOR WATER CONSERVATION: The larger lever is to flush out around 6 to 9 liters of water, whereas the smaller lever is to flush out around 3 to 4.5 liters of water. Clearly, the larger one is to flush solid waste and the smaller one is to flush liquid waste.

https://en.wikipedia.org/wiki/Dual_flush_toilet