Listings do not count in vale, only to make sure it is in line with market. You can list your home for a million $, the selling price is what is important.
Also the price per sq foot is more for the smallest home in the neighborhood. The largest would be the least per sq foot. IE If the 3 beds were 1000 sq feet and the 4th was 1200 sq feet. There would be a difference but not 20%. They should be taking the largest other comp (not larger than yours), and using that for the cost per sq foot.
The larger the home, the less 200 sq feet makes. IE if you 1200 instead of 1000, it may make a diff of 8K- 10K, but if you are are 2000 instead of 1800, maybe only 5K-6K, and or 2800 instead of 2600, maybe 3K. Over 3K, it would make no real difference. Generic #s of coarse.
They also price into account what people are buying. IE if the market is supporting the smaller homes, with lower prices selling (which it is in most areas), or foreclosures selling. Having the largest home on the block, may not actually increase your value much at all. The market may not be allowing more $$ for larger homes right now. Sometimes having the smallest home in the bigger neighborhood, is better than the biggest home.
Working in the mortgage field for 8 yrs, there is very little change of getting a value appeal approved. You can try, or even try w/ another lender (unless you are doing FHA then same appraisal for a yr anyway), but chances are it would be very close to the same.
Good news, if the appras is less than your taxed assessed value there is a shot you can argue your taxes down. You have a legal doc stating the value of your home is X. Possibly saving you more.