It's not just that, its all the fixed losses in the engine - friction and all the belt driven gubbins attached to it.
Think: if you car burns a gallon an hour *not moving at all*, until the aero losses are *also* a gallon an hour, you haven't reached optimum speed.
But as I said 70mph is about 1800rpm in top gear. I need to be rather faster than 40mph to GET to top...the auto box seldom lets the RPM drop below 1200...that is roughly where No 1 turbo kicks in..
There seems to be remarkably little difference between about 50 and 70mph. A little car with a small engine has very small fixed losses. Ergo it will have a much lower optimal speed.