“Cleaning up the tax code” and “reducing the size of government" aren't really the same thing; the former could very easily serve the opposite purpose of the latter.
Or, not; alternatively, it would allow the IRS to be more responsive in its supportive activities and more effective in its enforcement activities.
Cutting staff is a separate decision from streamlining the rules; the IRS hasn't had staffing expanded to keep workload constant when we weren't simplifying, there is no inherent reason it would ha r to cut staff and keep workload constant if we were simplifying.