iOS Mask Image size too high

I wasn't sure how to name this, sorry for the title.

I have an image that I want to fill more or less depending on a variable. For this, I created an image made of black and white (below). The goal is to use it as a mask and fill it the way I want, based on this documentation.

The issue: the image is drawn properly BUT its dimensions are way too high. I test it on iPhone 6+ with a @3x image, the image asset size is correct but the image that is returned by my function is way too big. When I put constraints on my UIImageView*, I only view part of the returned filled image; when I don't, the image is way too big. See below for a screenshot (iPhone 6+)

I subclassed a UIView with the following code:

- (void)drawRect:(CGRect)rect
{
CGFloat width = self.frame.size.width;
NSLog(@"%0.f", width);
CGFloat height = self.frame.size.height;
NSLog(@"%0.f", height);
CGFloat fillHeight = 0.9 * height;
NSLog(@"%0.f", fillHeight);

CGContextRef context = UIGraphicsGetCurrentContext();


CGRect fillRect = CGRectMake(0, height - fillHeight, width, fillHeight);
CGContextAddRect(context, fillRect);
CGContextSetFillColorWithColor(context, [UIColor yellowColor].CGColor);
CGContextFillRect(context, fillRect);

CGRect emptyRect = CGRectMake(0, 0, width, height - fillHeight);
CGContextAddRect(context, emptyRect);
CGContextSetFillColorWithColor(context, [UIColor blueColor].CGColor);
CGContextFillRect(context, emptyRect);
}

- (UIImage *)renderAsImage
{
// setup context
UIGraphicsBeginImageContextWithOptions(self.bounds.size, NO, 0.0); // use same scale factor as device
CGContextRef c = UIGraphicsGetCurrentContext();

// render view
[self.layer renderInContext:c];

// get reslting image
UIImage *result = UIGraphicsGetImageFromCurrentImageContext();
NSLog(@"result size = %d width, %d height", result.size.width, result.size.height);
UIGraphicsEndImageContext();
return result;
}

And in my ViewController:

- (void) setUpMaskedImage
    {
    // Image View set in Storyboard that will contain the final image. Bounds are set in Storyboard (constraints)
UIImageView* imageView = self.containingImageView;

// Custom View (see methods above) that will draw a rectangle filled with color
UIView* view = [[CustomView alloc] initWithFrame: imageView.frame];

// Mask Image used along with the custom view to create final view (see image above)
UIImage* maskImage = [UIImage imageNamed: @"maskImage"];

[view setNeedsDisplay];
UIImage* fillImage = [view renderAsImage];

CGImageRef maskRef = maskImage.CGImage;

CGImageRef mask = CGImageMaskCreate(CGImageGetWidth(maskRef),
                                    CGImageGetHeight(maskRef),
                                    CGImageGetBitsPerComponent(maskRef),
                                    CGImageGetBitsPerPixel(maskRef),
                                    CGImageGetBytesPerRow(maskRef),
                                    CGImageGetDataProvider(maskRef), NULL, false);

CGImageRef masked = CGImageCreateWithMask([image CGImage], mask);
UIImage* imageToDisplay = [UIImage imageWithCGImage:masked];
[imageView setImage:imageToDisplay];
}

I just don't get it. I used similar code elsewhere in my app and it works just fine. I will do a sample project soon if necessary.

Thank you!