I just threw this together for an ARIN IP address allocation policy conversation regarding the IP (v4, v6 aren't a problem) address blocks needed by Internet exchange points.
#IXP The issue is that all IXPs start small, and it's hard for people who are trying to get three ISPs to stay in the same room long enough to come to agreement to imagine that some day, they might have more than 256 ISPs all participating... yet 37 have passed the /24 threshold already, eleven have passed /23, three have passed /22, and one (PTT São Paulo, FTW!) has passed /21.
Renumbering an IXP is a whole different beast than renumber an ISP or enterprise. Instead of a top-down decision that gets executed within a single organization, every participating network has to renumber their peering router and reestablish their BGP peering sessions. So the larger the IXP, the more daunting the task, and the more organizations have to be wrangled.