First issue: you've predeclared segment_label_images and pixelCount however, you don't predeclare the matrices in each cell of segment_label_images, so these grow at each step of the i and j loops.
Workaround: before the i loop:
for k = 1:numerofClasses
segment_label_images{k} = zeros(size(classImage));
for i = 1:rs
%...
However, there's no point in the i and j loop, and thus actually no need to predeclare the matrices:
for k = 1:numberofClasses
segment_label_images{k} = classImage == k;
pixelcount(k) = sum(segment_label_images{k}(:));
figure;
imshow(segment_label_images{k}); %no need for []
end
If you need the segment_label_images, there's no way to avoid the k loop (or arrayfun). If you only need pixelCount, then:
pixelCount = histogram(classimage, 1:NumberOfClasses, 'BinMethod', 'integers')
