They have to draw the line somewhere and the average size of a family worldwide is 3.4 members.
So they already doubled the worldwide average for the system.
There will always someone be who has a bigger family, we already had people with 20+ family members who want the limit to be raised.
In the end this system can not make everyone happy and some people will be left out.

If you have enough family members maybe create a second family for them.