雷声大雨点小（léi sheng dà yǔ diǎn xiǎo）

雷声大雨点小（léi sheng dà yǔ diǎn xiǎo； thunder sounds loud, but rain drips are small）

The thunder is loud but the raindrops are small. This idiom can be used to describe a situation in which much is said but little is done.

This is similar to English expression: great boast, small roast.