big
駉 centers on largeness, specifically applied to horses or livestock. The single given meaning 'big' is its core idea, but the kanji is rare and its exact nuance is uncertain.
駉 combines 馬 (horse) with 冋, which likely contributes the sound. The exact historical development is uncertain, but it probably originally described a large horse.
A horse 馬 beside a wide enclosure 冋: picture a massive horse too big for its stable, emphasizing its size.
For ケイ, imagine a big horse named Kay: Kay -> ケイ, and the horse's size matches the meaning 'big'.