Imwarp results in unwanted image resize

I'm trying to correct for keystone distortion for a flat object imaged by the camera with incidence angle about 15-20 degrees. So, I shot several checkerboard images and obtained camera parameters with estimateCameraParameters function. Then I determine the transformation from one of the checkerboard images, invert it and use imwarp to obtain the image as if it was taken orthogonally to the object. And I'm very surprised to see that, whereas the original image im is 3456x5184x3, the transformed image imNew is only 186x317x3. Is it possible to keep the original image resolution? Here is the code, MyCamParams.mat contains camera parameters cameraParams determined by calibration.

load('MyCamParams');
[workimname, workimpath]=uigetfile('*.jpg');
im=imread(fullfile(workimpath, workimname));
[imagePoints, boardSize]=detectCheckerboardPoints(im);
squareSize=10; % in millimeters
worldPoints=generateCheckerboardPoints(boardSize, squareSize);
[R, t]=extrinsics(imagePoints, worldPoints, cameraParams);
T=[R(1, :); R(2, :); t]*cameraParams.IntrinsicMatrix;
tform=projective2d(T);
imNew=imwarp(im, invert(tform));