Swift CollectionView changes size when imageview is inside

I have created a CollectionView with three items per row. If my cell includes just a view its working really fine like the picture shows:

But if I add an Imageview inside my view with equal constraints its going to look like this:

Here is my Code for my CollectionView:

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
    return UIEdgeInsets.zero
}

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumInteritemSpacingForSectionAt section: Int) -> CGFloat {
    return 0
}     

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
    let yourWidth = collectionView.bounds.width/3.0
    let yourHeight = yourWidth

    return CGSize(width: yourWidth, height: yourHeight)
}