Person figure underneath — body and legs viewed from the front. Strict etymology says 大 is one pictograph of a frontal-stance person, but visually it splits cleanly into 一 + 人.
Radical
BigKangxi #37
The 大 radical, a pictograph of a person with arms outstretched. Indexes characters about people, scale, or magnitude: 天, 太, 央, 夫, 头, 奇, 套, 奖. Common as the top component but a frequent semantic anchor wherever it appears.