(I thought this was scheduled to pop automatically yesterday, but… it wasn’t, and I was remiss in failing to check. My apologies for the delay.)
Literally: gall bladder / courage – big – heart / mind – small
Alternately: Not being fearful when it comes to taking action, but also paying attention to the fine details. Both bold and careful; proactive and meticulous.
Notes: A variant replaces 小 with 細 (sai), “small (detail).”
This compound comes to us from the Old Book of Tang (『旧唐書』, in Japanese Kutoujo), a history form the “Five Dynasties and Ten Kingdoms” period about the Tang era.