秤砣雖小壓千斤 [－－虽－压－－]

Jyutping cing3 to4 seoi1 siu2 aat3 cin1 gan1
Pinyin chèng tuó suī xiǎo yā qiān jīn

Definitions (CC-CEDICT)
  1. although small, a steelyard weight may tip a hundred pounds (idiom)
  2. apparently insignificant details can have a large impact
  3. for want of a nail the battle was lost