Ryanair do allow pax to buy more than one seat for themselves, all you have to do is speak with their call centre.
I know this, because I have done it on a +3 hour sector, to get enough space to use a laptop.
The nice aspect of the Southwest scheme is that the extra ticket is refunded if there is a spare seat.
Now one can argue that the large person is getting a better deal than the other pax, but personally I don't begrudge them that - life is tough enough when you are oversized.