gate, barrier, wide, vast, expand
閎 centers on a large, imposing gate or barrier that is wide and expansive. The idea of vastness extends naturally to expanding or opening up space.
閎 combines 門 (gate) with 厷, which likely contributes the sound and may relate to the idea of largeness or stretching. The exact historical development of 厷 within this character is uncertain.
The gate 門 surrounds a shape 厷 that looks like a wide, stretched arm. Imagine a massive gate so wide that it takes an outstretched arm to span it.
For コウ, picture a wide gate at the entrance to a grand hall where a 公 (こう, public) event is held: 公 -> コウ, and the gate is vast enough for everyone.