It already does.
It's just very faint, so it's hard to see under normal conditions.
But the Andromeda galaxy is as big, if not bigger, than the apparent diameter of the Moon in the sky (about half a degree of arc.)
I'm on an astrophotography site and many novices complain that when they point their telescope at M31 and shoot some pictures, all they see is a bright blob - they don't realize that that's because they have so much magnification they can only see the core.
With the naked eye, how big should M31 appear? I have tried and tried and have never been able to spot it. (I do know exactly where to look, and have even tried using my peripheral vision). I have often suspected that I don't know what I should be looking for size-wise. I presume it is brightest at its centre, thus its apparent size will be smaller because I'd only be seeing the bright core.
With a telescope - unless you have A LOT of aperture - all you are gonna see is the core.
With binocs and a good sky, it's whole other story
Starry Night puts it at a little over 2 degrees across - but I was at a Star-Camp event last week, in Kielder Forest - generally regarded as England's darkest spot - and I could see the faint haze of the disk, extending as wide, possibly even WIDER than the FoV of my SkyMaster 15x70 binocs (4 degrees FoV) :surprised
Naked eye - it appeared as wide as the gap between 'nu' and '32'Andromeda, immediately below it.
BTW - If anyone is interested, I did a bit of a write-up of my own experience of the Kielder event for my own little forum community...
M31 is listed at 4th magnitude, so you'll need pretty dark skies to pick it up at all with the naked eye and when you do, it'll be barely more than a fuzzy star. If you look at the photo in the link, you can see just how much brighter the core is than the outer areas - and that's probably with unequal stretching of the brightness (in my pictures, I can't get it that balanced). So the outer starfields are nowhere near bright enough to see naked eye.
If I can find it when I get home, I have a photo taken with a standard digital camera at no magnification that shows it....
Actually, for now, take a look at my photos of it on my website (shameless plug) - check out how washed-out the core is in the first one. Try looking at the thumbnail from 20 feet away... http://www.russsscope.net/deepspace.htm