Not at all. If I remember correctly iPhones will download the latest version of iOS over WiFi when it is released as long as you have enough space available.
So say Apple gets rid of the WiFi restriction and then releases an iOS update that is 120MB. As soon as they release that update every iPhone that is out there (with free space) would start downloading it. That is millions and millions of iPhones.
Compare that to an app update that is 120MB. What are the chances that every iPhone out there has that app installed, and has auto update turned on? The vast majority of apps are only going to be on a fraction of the phones out there, and even out of those many people would likely have auto update turned off. So even if both updates are the same 120MB, the iOS update is going to get downloaded by a much larger amount of people at the exact same time, and put a much larger load on the cellular networks.
There would be a few apps like YouTube and Facebook that are on a large percentage of phones, so those apps could come close, but still not hit the networks as hard as an iOS update.