I believe the smaller families are due to all the above factors. Miscarriages, high cost of living, getting married later, lower fertility, over use of NFP, some catholics unfortuanatly use birth control.
Im expecting #6 and that is alot of work but is their anything compared to life itself? So I keep going. I have read though as Catholics there are 3 major reasons to space your children and even then the option is abstence.
#1 Due to major health problems
#2 Due to emotional/physicology problems
#3 Due to major financial problems
Those are the only 3 I am aware of for keeping the families small if you are a person who is able to conceive and is of course married.