I did some experiments dragging various sized stones across my lawn with a spring balance to measure the force required. It turned out to require a horizontal pull of something like 80% of the weight of the stone to keep it moving once it was started. So proportionally a 40 ton stone would need about 32 tons to drag it over grass.

A man pulling can easily produce 50kg of pull, so it would need about 20 men per ton or around 640 men to drag a 40 ton stone.

The reason why very large stones present an apparently disproportionally greater problem that small stones is that the ground pressure increases linearly with the thickness of the stone and since big stones tend to be thicker than small ones, there is a greater tendency for the stone to "peck", i.e. dig into the ground. This causes problems in getting it moving in the first place and also tends to increase the perceived frictional forces.

However, pecking can be reduced by several methods. Firstly, harder ground helps, so a prepared roadway (or frozen ground) is a possibility. Secondly, a sledge can be used to spread the weight over a larger area and hence reduce the ground pressure. Thirdly, a shallow rockered front either on the stone itself or on the sledge will help to prevent pecking.

My limited experiments with lubricants suggest that cow dung dribbled in front of the stone would produce a dramatic reduction in the friction, possibly reducing the required workforce by up to two thirds.

The Egyptians could move stones of over 1,000 tons and their depictions illustrate people dragging with ropes while others levered from behind.

One of their pyramid texts shows a huge statue (probably weighing several hundred tons) being dragged on a sledge and there is a man riding on the front of the sledge pouring something on the ground. I suspect in this case the roadway has been surfaced with Nile mud that has been left to dry and the man is pouring water onto it to turn it slippery again once the pullers are clear.

It's fairly clear that stones much bigger than the sarsens can and HAVE been moved my sheer brute force as Nigel maintains.