There is a philosophical principle call Occam's Razor that says of two possibilities, the simpler one is the more likely to be the correct one. With all that we know of physics, it is much more reasonable to suppose that space is expanding and carrying its contents with it than that everything in a fixed space is shrinking. There is no plausible basis for a supposition that matter is shrinking, whereas the Big Bang theory offers a plausible basis for the Universe to have started from an extremely hot concentration of energy that was converted into extremely energetic particles of mass via Einstein's E = mc^2 as it cooled.