Turfnut is right. Always know your costs to do the job. Big lawns require just as much fertilizer per 1000 sqft as small lawns. And the labor is about the same per 1000 sqft, (say 1000 sqft per minute). So the only reason to discount a big lawn is there is a slight saving on trip cost--only one trip. And only one stamp to send the bill.
A big company in my area 2 years ago charged $22 for the trip and overhead cost...and then charged $6 per each 1000 sqft. However their salesmen sometimes under measured their lawns.