Goats reach their peak yield about 6 to 8 weeks after kidding so she could be doing over 3 kg a day by then. A very rough estimate of her lactation yield (300 days) is about 800 kg. That's a lot of milk for an animal that only weighs 70kg. If you compare this estimated yield with that of a Holstein / Friesian heifer weighing 550 kg and producing 5,000 kg in her first lactation the goat is 20% more efficient weight for weight. But then as someone famously said, "there are lies....., damn lies and... statistics".
I should add; goats milk is creamy and delicious. There's no "goaty" flavour and we are all enjoying it.