Never tried it, but I'd imagine there's a heck of a lot of practice goes on to work out the most efficient way to do this. Trouble is, the human body is still not that efficient at getting into small spaces especially when bent as it has to be in these cases so there's a lot of free air space.
Crushing them isnt too much of a problem simply because you've usually got 1 person at a time pushing. If there's physically not enough space, it's very unlikely that person will be able to create enough force to crush several other people which would be required to crush those in the middle. Also the way they have to bend means that the forces are distributed throughout bending bodies. If they were laying flat it would be a lot more dangerous.
Crush injuries are much worse when everyone is aligned the same way, eg: standing up. That has been shown many times such as the Hillsborough Disaster where 100's of people were pushing from behind and all the people were standing up (equiv. of laying down).