We've been downsizing ever since I got here. I haven't lost storage capacity yet though--because I did a careful evaluation when I arrived and determined that our flatfiles were 200% full. So even though we've weeded something like 60,000 sheets (most topos, but there's been a lot of de-duping), thus far our upcoming renovation doesn't force me to throw away any maps cases.
Fewer and fewer places are keeping nation-wide topos.
As for process and advice: I think we've all been depending on the Force to guide us down to the size that our institutions are willing to make us. I do fear, especially knowing how many collections, like mine, are not fully cataloged, that there are babies in that there bathwater.
-j