undistortFisheyeImage outputs much smaller image than original image

Dear all,

I am calibrating a GoPro Hero 5 black following the MATLAB example provided at the function undistortFisheyeImage(). I moved the camera around a checkerboard and analysed 10-200 frames from the video (variations in number of frames didn't change the outcome much). I then used the following code

% Calibration name
calname = 'calGoProHero51080p60fpsWide.mat';

load(calname)

images = imageDatastore('myPath\calibration1080p60fpsWide\','FileExtensions','.jpg');

[imagePoints,boardSize] = detectCheckerboardPoints(images.Files);
squareSize = 20; % millimeters
worldPoints = generateCheckerboardPoints(boardSize,squareSize);
I = readimage(images,1);
imageSize = [size(I,1) size(I,2)];
params = estimateFisheyeParameters(imagePoints,worldPoints,imageSize);

I = readimage(images,20);
J1 = undistortFisheyeImage(I,params.Intrinsics);
figure
imshowpair(I,J1,'montage')
title('Original Image (left) vs. Corrected Image (right)')

J2 = undistortFisheyeImage(I,params.Intrinsics,'OutputView','full');
figure
imshow(J2)
title('Full Output View')
save(calname,'params');

The calibration for a typical image looks like this (e.g. 1080 Wide view):

However, for a different camera setting (1080p Narrow view), I expect less distortion and for the rest a similar image. Unfortunately the image becomes very small as shown here:

I took a different calibration video; again moving the camera around the calibration checkerboard but the result stays the same. Can anyone shed light on this?

Kind regards,

Ernst Jan