It looks like there is a black path separating the different regions. I think you can just binarize the image and use bwareafilt
% Binarize the image
binaryImage = yourImage > 0;
% Make sure it's 2-D, not 3-D (which would be the case if yourImage was RGB).
if size(binaryImage, 3)) == 3
binaryImage = binaryImage(:, :, 1);
end
binaryImage = bwareafilt(binaryImage, 1); % Take largest 8-connected region.
If you get multiple regions (because regions touch on a diagonal), then try using 4-connected:
binaryImage = bwareafilt(binaryImage, 1, 4); % Take largest 4-connected region.
