I've desperately searched for an answer to this as well, and I even found these two threads on Microsoft's official forums for this kinda stuff:
Notice how none of the techie (so-called "engineers") do absolutely nothing to address the real problem and instead provide the copy+paste instructions for how to change icon size. One of them even says to try doing a "clean boot" which is totally pointless, because this is a problem with Vista even before you begin to tinker with settings and installing 3rd party software.
For what it's worth, here I'll copy my own report of this problem here that I had posted in one of those threads:
Here is what I think part of the problem is: there are actually 3 ways to change icon size, which are the two listed above and also one other in the Advanced Appearance Settings.
With three things vying for control over desktop icon size, it's no wonder that Vista will "hiccup" once in a while and temporarily reset the icon sizes to look smaller.
When this "hiccup" occurs, give the desktop focus, hit F5 to refresh the desktop and voila, the icons will go right back to the size you had set them at.
The "hiccup" will keep happening at random times when the Explorer process seems to kinda "refresh" itself... I'm not sure what is really happening, but that's how it appears to me. So, no matter what you keep trying to force the icon size to, it will occasionally reset to a smaller size (temporarily until you either restart, hit F5, or force another size change).
I've experienced this in Vista Home Premium 32bit and Vista Ultimate 32bit, and I've found zero solutions to this. Only a few other people have had the brains and guts to notice this and actually post about it, it's such an insignificant problem.
I tripple dog double quadruple dare you to find out why this glitch keeps happening. Beat up a couple of MS techies if you have to, it's worth it.