i-adjective
Colloquial, slightly childish or affectionate version of 大きい. Often used in casual speech, especially with children or in friendly contexts.
See also: ちっちゃい
わあ、おっきい犬だね!
Wow, that's a big dog!
このケーキ、おっきいからみんなで食べよう。
This cake is big, so let's all eat it together.