UIScrollView上でズームした時のUIImageViewをセンタリングorリサイズしたい

UIScrollViewの上にUIImageViewを乗せてピンチで縮小/拡大とダブルタップで最大倍率/最小倍率
というズームをしているのですが、画像の高さが小さい画像でズームすると
下の写真のように、何もない領域を表示できてしまいます。
試行錯誤してみましたが解決策がわからず質問させて頂きました。

解決策のご教授お願い致します。

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画像

[自アプリズーム前]......(問題発生)[自アプリズーム後]......(目標)[デフォルト写真アプリズーム後]

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現状

1. AutoLayout使用なし
2. view contentMode →scaleToFill
   scrollView contentMode→scaleToFill
   ImageView→AspectFit

3. self.view.frame == self.scrollview.frame == self.mainImageView.frame;
   三つとも同じフレームで被せています。

   ※RMP~はナビゲーションバーを使ったカスタム画面移動のOSSです。
   RMPZoomTransitionAnimator/GitHub

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当該コード

< DetailViewController.h>

#import <UIKit/UIKit.h>
#import "RMPZoomTransitionAnimator.h"
@interface DetailViewController : UIViewController<RMPZoomTransitionAnimating, RMPZoomTransitionDelegate>

@property (weak, nonatomic) IBOutlet UIScrollView *scrollView;
@property (nonatomic, weak) IBOutlet UIImageView *mainImageView;

@end

< DetailViewController.m>

#import "DetailViewController.h"
#define DEFAULT_ZOOM_SCALE 1.0
#define SCALE_RATE 3.0

@interface DetailViewController ()<UIScrollViewDelegate>
//zoom when doubleTap.
- (void)onDoubleTapOnPhoto:(id)sender;
@end

@implementation DetailViewController
- (void)viewDidLoad {
    [super viewDidLoad];
    self.scrollView.minimumZoomScale = DEFAULT_ZOOM_SCALE;
    self.scrollView.maximumZoomScale = SCALE_RATE;
    self.scrollView.delegate = self;

    UITapGestureRecognizer *doubleTap = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(onDoubleTapOnPhoto:)];
    doubleTap.numberOfTapsRequired = 2;
    [self.view addGestureRecognizer:doubleTap];


    // Do any additional setup after loading the view.
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}
- (void)onDoubleTapOnPhoto:(id)sender
{

    float zoom = self.scrollView.zoomScale;

    CGPoint tapPoint = [sender locationInView:self.scrollView];

    // Zoom=self.scrollView.zoom → Zoom=1.0
    tapPoint.x /= zoom;
    tapPoint.y /= zoom;

    if (zoom >self.scrollView.minimumZoomScale)
    {
        // already max zoom : set zoom to default
        [self.scrollView setZoomScale:DEFAULT_ZOOM_SCALE animated:YES];
    } else
    {
        // zoom up

        float newZoom = zoom * SCALE_RATE;
        if (newZoom >= self.scrollView.maximumZoomScale)
        {
            newZoom = self.scrollView.maximumZoomScale;
        }

        CGRect newRect = [self zoomRectForScrollView:self.scrollView
                                           withScale:newZoom
                                          withCenter:tapPoint];
        [self.scrollView zoomToRect:newRect animated:YES];
    }
}

#pragma - scrollViewZooming
- (UIView *)viewForZoomingInScrollView:(UIScrollView *)scrollView
{
    return self.mainImageView;
}

- (CGRect)zoomRectForScrollView:(UIView *)scrollView withScale:(float)scale withCenter:(CGPoint)center {

    CGRect zoomRect;

    // The zoom rect is in the content view's coordinates.
    // At a zoom scale of 1.0, it would be the size of the
    // imageScrollView's bounds.
    // As the zoom scale decreases, so more content is visible,
    // the size of the rect grows.
    zoomRect.size.height = scrollView.frame.size.height / scale;
    zoomRect.size.width  = scrollView.frame.size.width  / scale;

    // choose an origin so as to get the right center.
    zoomRect.origin.x = center.x - (zoomRect.size.width  / 2.0);
    zoomRect.origin.y = center.y - (zoomRect.size.height / 2.0);

    return zoomRect;
}

#pragma mark - <RMPZoomTransitionAnimating>

- (UIImageView *)transitionSourceImageView
{
    UIImageView *imageView = [[UIImageView alloc] initWithImage:self.mainImageView.image];
    imageView.contentMode = UIViewContentModeScaleAspectFill;
    imageView.clipsToBounds = YES;
    imageView.userInteractionEnabled = NO;
    imageView.frame = self.mainImageView.frame;

    return imageView;
}

- (UIColor *)transitionSourceBackgroundColor
{
    return self.view.backgroundColor;
}

- (CGRect)transitionDestinationImageViewFrame
{
    return self.mainImageView.frame;
}

#pragma mark - <RMPZoomTransitionDelegate>

- (void)zoomTransitionAnimator:(RMPZoomTransitionAnimator *)animator
         didCompleteTransition:(BOOL)didComplete
      animatingSourceImageView:(UIImageView *)imageView
{
    self.mainImageView.image = imageView.image;

}
@end