unit of weight, small, slight
錙 centers on a small unit of weight, extending to the idea of something slight or insignificant.
錙 combines 金 (metal) with 甾, which likely contributes the sound. The character originally referred to a small metal weight unit.
The left side 金 (metal) suggests a metal weight, and the right side 甾 looks like a small container. Imagine a tiny metal weight used for precise measurements, so small it seems slight.
For シ, picture a tiny weight so small it looks like a single seed (シード): seed -> シ, and the metal weight is as light as a seed.