West Ham moved from their ground at Upton Park to the much bigger London Stadium, and they haven’t felt at home while playing in their new home ground.
The Hammers only lost three games at home last season but have lost way more than that figure in the London Stadium this season.
Costa believes it’s the difference in size between Upton Park and the London Stadium, and Chelsea were very comfortable playing there.
He told ESPN Brazil: “It was always a tough game. When they come to a bigger pitch, with the crowd further from the players, maybe they don’t feel the same heat from the crowd.
“For us or other teams that come here it’s almost like coming to a neutral ground, because no one’s used to be playing here. It’s more equal.”