The tax administration is far more efficient than that. The IRS has 79K workers out of a total workforce of 158M, or 1/2000 workers. Federal taxes are about 19% GDP (28% of GDP including state and local taxes.) The IRS costs $14.3B to run and collects 19% of $25.46T = $4,800B or 0.3% of collected taxes.
> If IT really improved productivity, wouldn't you expect that that number would decrease, since Tax Administration is presumably an area that we should expect to see great gains from computerisation?
Those gains are so great that tax administration has been computerised for more than half a century now.
We could certainly save an awful lot more in tax preparation for the economy as a whole if we sent out pre-filled tax forms (the IRS already has all the information required for most people) like other countries do but the tax preparation companies have made a lot of contributions to politicians to prevent this.