It's what I thought, too. The split may come from the fact that the front-end is more complex, richer in information, harder to analyze, and harder to transform. The more you can do with it the more code it takes to represent. That's my hypothesis.

Far as Ada, I figured it would be difficult due to the large number of language features and complexities of analyzing a program for safety w/ all its rules. And any interactions that might emerge in that between features and rules. Could be easier than I think but my default belief is that it's not easy.