Windows 8.1 Update 1 doesn't sound like a very big update to me. It should certainly be a smaller update than 8.1, if they didn't name it 8.2, and there was nothing major about the 8.1 update to begin with, unless you consider the adding of a Start Screen shortcut button a "major upgrade".

Windows is a behemoth that needs to support a ton of different hardware configurations, and it can't come out until it works very well on almost all of them. That means a year of development is a very short amount of time for Windows, and they can't add too much to it. Look at Windows 7 vs Vista. It took 3 years just to improve its performance a bit.