Hi nicobean - regarding larger families in the UK vs USA: I think it depends on how you run your life. Yes, houses tend to be bigger/less expensive (especially if you are not talking London/NYC prices), and there is Costco, Target, etc.
But, my impression is that the Americans (especially suburban families) tend to have their dc in many more activities - so you can spend your life as taxi driver going from football/soccer to music lessons to dance class to swimming, etc. And that is before you've done a single birthday party or playdate. And since space is not at such a premium, the places you will be going are often not near by so alot of time in the car is likely.
I know these things happen in the UK also
but somehow the Americans seem to take their extracurriculars very seriously so weekends especially can be quite hectic with endless swim meets, 'travel' soccer games, basketball, weekend music seminars/workshops, etc. All those extras would be logistically hard with a larger family - unless you rideshare with other families, rope in grandparents to assist (which you wouldn't have here?), divide responsibilities with dh, or hire someone to be an extra set of hands/extra transportation (which some families in dd's class with 4 dc have done).
Qualifying statement: my experience was all in London (so don't know about routines outside), and we moved to America when dd was 7 (so before she was old enough to be 'serious' about any extracurriculars).