Why is iCloud Calendar data usage so high on my iPhone but not my iPad?

Thanks, but that's not a particularly helpful answer. My issue isn't that it uses cellular data, and I wouldn't want to disable the feature entirely in that way anyway.

My issue is that it uses an unreasonably large amount of data (on either cellular or WiFi as applicable) for what it's doing. Here is a quote from the article you linked:

"Experts believe that with Apple's use of Wi-Fi, plus the low-bandwidth nature of some of the updates, it shouldn't be a problem for most consumers. For one, much of the data that will be transferred between devices will be contacts, e-mails, calendar updates, and other text-based data that doesn't gobble up a lot of bandwidth. What's more, when devices are synched, they'll be updating only new information." [emphasis added]

And my biggest concern is that it uses ten times as much data on the iPhone as it does on the iPad to do the same thing.

In other words, something actually appears to be working incorrectly on the iPhones and it's working properly on the iPad. Like I said above, same set-up, exactly same calendars shared, etc.

I believe I've figured out most of what was happening.

On the iPad I had Settings > Mail, Contacts, Calendars > Calendars > Sync set to Events 1 Month Back. On the iPhone 4 it was set to All Events.

Even though I was only ever adding or changing one or two appointments, for some reason having this setting set to All Events seems to trigger a data transfer of about 4–5MB. This shouldn't be the case, because, as the article above points out, it should only be the new information that gets transfered. So I suspect a defect in the sync algorithm. Perhaps it can't handle the 9900 events in my calendar. But anyway, after changing the setting to Events 1 Month Back, the data usage for a change to an event went down to a much more reasonable 0.2MB. With the setting set to Events 6 Months Back, the data usage for the calendar update was about 0.3MB.

Finally, I purged all data prior to 2010 by moving it to a different calendar (which will not be in iCloud), so that there are now about 1280 items in the calendar instead of 9900. I then changed the sync setting back to All Events. Now calendar updates trigger a data transfer of about 0.6MB. This will still add up to a noticeable amount if there are a lot of calendar updates in a day, but it's perfectly acceptable.

So, in summary, for some reason iCloud can't properly handle syncing 10,000 events, and ends up transferring a ton of data for each calendar update, and the updates also didn't seem to get pushed properly in real time, or at least they took a long time.

After purging the calendar the amount of data transferred for each update is reasonable and the updates are now nice and fast.