Several parts combine into one character.

太 is a brilliant minimal differentiation: take 大 (big) and add a single dot underneath. That one dot transforms "big" into "biggest / extremely big / excessive." Originally 太 and 大 were the same character, but the added dot crystallized a separate meaning: ultimate, supreme, too much, fat. The single-dot difference between 大 and 太 is one of the smallest visible markings to carry the largest semantic shift in the entire CJK system.

Korean reading "tae." Korean preserves the "ultimate / supreme / origin" sense most strongly: 太陽 (taeyang, sun — "the supreme bright one"), 太極 (taegeuk, the supreme ultimate — the symbol on the South Korean flag, representing yin and yang in harmony), 太平洋 (taepyeongyang, "supreme peaceful ocean" = Pacific Ocean), 太古 (taego, "supreme ancient" = primordial / antiquity), 太子 (taeja, crown prince — the supreme heir).

Mandarin tài, 4th tone. Mandarin took 太 in a different direction: it became one of the highest-frequency adverbs in everyday speech, meaning "too / very / excessively." 太好 (tài hǎo, "so good"), 太多 (tài duō, "too much"), 太累 (tài lèi, "too tired"), 太贵 (tài guì, "too expensive"). Almost any feeling or judgment in Mandarin can be intensified with 太. And 太太 (tàitai, "Mrs. / wife") doubles the character to mark a respectable married woman — perhaps influenced by the original "great / supreme" sense applied to a household authority.

Japanese has two on-readings. タイ (tai) — 太陽 (taiyō, sun), 太平洋 (taiheiyō, Pacific Ocean), 太古 (taiko, primordial era), 太鼓 (taiko, drum — the large traditional Japanese drum, named for its "thick / fat" body). タ (ta) is rare and appears in compounds like 丸太 (maruta, log — though this is a hybrid kun + on reading). Kun-reading ふとい (futoi, "thick / fat") — 太い線 (futoi sen, a thick line). When applied to people: 太っている (futotte iru, "is fat / has gained weight") — a polite-ish way to describe weight in Japanese.

Memory aid: 大 (big) plus one dot underneath — that small mark is the difference between "big" and "biggest / too big." A single dot of intensification.