雷聲大，雨點小

雷声大，雨点小
léishēngdà,yǔdiǎnxiǎo
  1. lit. loud thunder, but only tiny drops of rain (idiom)
  2. fig. a lot of talk, but no action

CEDICT

Entry Composition

1 雷聲 léishēng thunder
2 大 dà
  1. big; large; great
  2. older (than another person)
  3. eldest (as in 大姐 dàjiě)
  4. greatly; freely; fully
  5. (dialect) father
  6. (dialect) uncle (father's brother)
3 雨點 yǔdiǎn raindrop
4 小 xiǎo
  1. small
  2. tiny
  3. few
  4. young