Simple is related to Ordinary. Ordinary is the opposite of Special. 
Decrease is the opposite of Grow. Grow is the opposite of Diminish. Diminish is a synonym of Decline. Decline is a synonym of Avoid. 
Simple <--RelatedTo--> Ordinary <--Antonym--> Special 
Decrease <--Antonym--> Grow <--Antonym--> Diminish <--Synonym--> Decline <--Synonym--> Avoid 