There is some similarity in the (hook) of the song when he says, "Want you stay with me" with Tom Petty And The Heartbreakers - "I Won't Back Down". "Stay With Me" almost sounds like a slowed down version of Tom Petty's song. Generally speaking these types of suits are difficult to prove. Therefore I'm incline to believe there must have been enough musical notes in line for the courts to say there is some copyright infringement.
Venus if you "don't know" how can you automatically say Sam Smith got the short end of the stick? or Are you really saying "you don't care"? If someone showed you notes on a sheet of music proving the songs are very similar would it matter to you?
It's not the other way around because Tom Petty wrote his music years before. He is asserting that Sam Smith stole or copied his musical notes composition. Apparently Tom proved it in court which is not an easy thing to do! Sam Smith will be fine