On the 16th century, an assistant would literally hold a candle to his superior by standing beside him with a candle to provide enough light for him to work by. The modern version suggests that the subordinate is so far inferior that he is unfit to perform even this humble task.
Related Idioms :
burn the candle at both ends
lavish energy or resources in more than one direction at the same time
go to bed late and get up early
not worth the candle
not justifiable because of the trouble or cost involved
The idea behind this idiom is that expenditure on a candle to provide light for an activity would not be recouped by the profits from that activity. The expression comes from the French phrase le jeu ne vaut pas la chandelle - the game is not worth the candle.
1998 - New Scientist - But what if, instead of one, five, fifteen or fifty people... have to endure such an existence? At what point does the game cease to be worth the candle?