Gaussian filter looks pixelated What does it mean when an image looks pixelated after a Gaussian filter (Multivariate Gaussian Distribution filter) is applied (in context of mu, Sigma, and the meshgrid created), and what parameters can one alter to minimize this effect and make the image look smoother?

Sample code (MATLAB):

d1 = 20
d2 = 20
d3 = 20
mu = [0 0 0]; 
Sigma = 30*[1 0 0; 0 1 0; 0 0 1];
[X1,X2,X3] = meshgrid(linspace(-10,10,d1)', linspace(-10,10,d2)', linspace(-10,10,d3)');
X4 = [X1(:) X2(:) X3(:)];
p = mvnpdf(X4, mu, Sigma);
F = reshape(p,d1,d2,d3);


I am convolving the filter above with a 3-D matrix, a roughly 90*60*100 matrix.
An image shouldn't look pixelated after filtered by a Gaussian (lowpass) kernel... instead look at the following figure: