little, small, tiny, petty
蕞 centers on extreme smallness or insignificance: something little, tiny, or petty. The meanings all point to a diminished scale or trivial importance.
蕞 combines 艹 (grass) with 最 (most/extremely), likely suggesting something as small as a blade of grass or the idea of being extremely tiny. The exact historical development is uncertain.
The grass radical 艹 sits above 最, which means 'most'. Imagine the tiniest blade of grass, the most small thing you can find: 蕞 is little and petty.
For サイ, picture a tiny sigh escaping from a blade of grass: sigh -> サイ, and the sound is as small as the kanji's meaning.
—