Applications parse crazy complex stuff to do everything they do, so obviously have a really big attack surface. Often the complexity is unavoidable - if you are a web browser, you cannot avoid parsing html for example.

However the sandbox is designed to have an attack surface as small as possible (and be configurable via permissions to have the bare minimum needed). The sandbox interfaces with the rest of the system are fully controllable by Apple, so there is no need to be passing complex and dangerous legacy datastructures across the boundary either.

Therefore, it should be the sandbox that is 'hardest' to break out of.