It seems odd to me they'd be so big, since I'm guessing a decent number of people that visit the front page use a regular 13-inch Macbook (being 1280x800, which isn't particularly small anyway) and half of the image gets cut off at both sides. I've noticed it with a few of them now. I wanted to see CNET's comment on iSlash and had to right-click and "View Background Image" to view the whole thing.
Same reason why the volume of commercials on TV are substantially louder than the show you're viewing, to blast you with marketing.
We don't design the site skins, we merely give advertisers the template for them to design their ad campaigns around.
1920x1080 for me so they are fine. I don't like the ads, let me rephrase that, I HATE THEM, but it's not up to me so I guess I'll deal with them.
Ads are fine, what's annoying is when I'm trying to browse the site on my iPhone and right when I'm about to click the 'New Posts' button the whole page jumps about 2 lines down forcing me to click one of the stupid banner ads. Happens roughly about 50% of the time.