There's a big difference between what's "comfortable to carry back by hand" and "what's feasible to carry to a bus stop 100 metres outside the store, and then 400 metres from where the bus drops you off to your home". That's if we're assuming a situation where you did drive to the store, planning to drive home, but an emergency means you can't drive the return leg.

But also, if you do have good public transport, it becomes much easier to adjust your schedule to more frequent, smaller shops, where it's not just feasible but easy to carry the groceries. Or in a good city for cycling, to drop the groceries in your paniers, basket, or even full-on cargo bike.