large, great, eminent
碩 centers on large scale and outstanding quality: something physically big, great in importance, or eminent in status.
碩 combines 石 (stone) and 頁 (head), originally suggesting a large stone or a head-like prominence, extending to the idea of greatness or eminence.
A stone 石 head 頁 is a massive, solid head—picture a giant stone statue with a prominent head, representing something large and eminent.
For セキ, imagine a great stone 石 (せき) head: the reading of 石 itself is セキ, so the large stone head directly cues セキ.