sky, big
昊 centers on the vast, open sky, often with a sense of expansiveness or greatness. The 'big' meaning is an extension of that boundless sky-like quality.
昊 combines 日 (sun) and 天 (sky/heavens), suggesting the sun in the vast sky. The exact historical development is uncertain, but the component combination clearly points to the sky or heavens.
The sun 日 sits above the heavens 天, painting a picture of the immense, open sky. When you see 昊, think of a sky so big it stretches forever.
For コウ, imagine a huge sky so vast it makes you say 'Whoa!' — the 'ko' in 'Whoa' cues コウ, and the sky is big enough to take your breath away.