UITableViewCell默认的imageView大小在滚动时更改

Question

以下是我的cellForRowAtIndexPath：

func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
    var cell:UITableViewCell!
    if cell == nil {
        cell = UITableViewCell(style: .Subtitle, reuseIdentifier: "cell")
        cell.backgroundColor = UIColor.clearColor()
        cell.selectionStyle = .None
        cell.accessoryType = .None
        cell.tag = indexPath.row
    }
    else {
        var arr = cell.contentView.subviews
        for i in 0..<arr.count {
            let view = arr[i]
            view.removeFromSuperview()
        }
    }

    let people = array_people[indexPath.row]
    cell.textLabel?.text = people.name
    cell.detailTextLabel?.text = people.info


    if let img = imageCache[people.image_url] {
        cell.imageView?.image = img
    }
    else {

        cell.imageView!.image = UIImage.gifWithName("loading")

        NSURLSession.sharedSession().dataTaskWithURL(NSURL(string: people.image_url)!, completionHandler: { (data, response, error) -> Void in

            if error != nil {
                print(error)
                return
            }
            dispatch_async(dispatch_get_main_queue(), { () -> Void in
                let image = UIImage(data: data!)
                cell.imageView!.image = image
                self.imageCache[people.image_url] = cell.imageView?.image
            })

        }).resume()

    }

    cell.imageView?.layer.cornerRadius = (cell.imageView?.frame.size.width)!/2
    cell.imageView?.contentMode = .ScaleAspectFit
    return cell
}  

下载图像后，我在滚动UITableView后回到它，它的大小已经改变。为什么会这样？