Top fused unit - historically
大 (great, supplying the sound dà to tài) merged with
廾 (two hands raised) into a single dam-shape silhouette. Standalone
大 has only 3 strokes; in
泰 it absorbed the
廾 hands to become 5 strokes. The original etymology was 'great-hands washing-water' - a person being rinsed by hands sluicing water, hence 'unimpeded, free, peaceful, grand.' Same phonetic family produced
太,
汰.