You can try using
xy=zeros([size(mov(1).cdata),length(mov)], 'uint8');
and change your imshow line to
imshow(xy(:,:,:,i));
Do not be surprised if this runs out of memory. Not matter how much memory you have, there will always be a movie large enough that you cannot make two fully-decompressed copies of it in memory.
What is it that you are going to do with your xy array? Is it certain that you cannot do the processing on the mov structure instead ?
