Okay, I think I figured out what problem you're having. I have Windows 7 dual-booted on my Mac, so while it's not 100% the same it's close enough. I changed my "Computer" icon to another of the already available icons, so my desktop looks like this:
I then tried both the Ctrl+Scroll and Right Click > View > Large Icons and got this:
So simply, I think the answer is that some icons just don't support going to a higher size level. As you can see not only did the Computer icon stay the same but so did Steam, and QuickTime. Other icons hidden by the browser window that you can't see were a RAR file and mIRC, which also didn't increase in size.