To be sure. And if we were doing routine maintenance on a longstanding codebase things might be a bit more predictable than they are. But we're trying to do new feature development on a new product, and my experience has been that beyond a very general sense of "this is going to be a big one" vs "this shouldn't take too long, unless I run into something unexpected" -- which is the same level of detail you get with or without Agile -- sizing isn't meaningful enough to be worth the time it takes to do.