I think when you logically conclude that passing a medium watermelon (for want of a better object) through a hole designed for a lemon some women will inevitably tear & some wont.
I think apart from the other things I mentioned, the psychological aspect can be a biggie too. If you think about the above comment in labour, you are only going ot be less reluctant to let the baby come freely as you are waiting to tear. One mum said that she envisaged a walnut coming through a toilet seat to relax the area and baby was born fine and well.

Also, if you take a look at the stats I posted for tearing / episiotomy for homebirth, it's a massive difference to that in hospital. So it's not something to expect to tear. Do your research and find ways to minimise intervention which relate to tearing and episiotomy. It's amazing that many, many midwifes with tens of years of experience never perform them or have had to perform them once. Doesn't this say something?