It depends what the question really is... if the appraisal is being done for a re-fi, the bank may only be willing to underwrite the home plus 5 acres - so comparing to other homes with 5+/- acres makes sense. Its not that the remaining land does not have value, but many loan programs will simply not accept acreage as collateral for a standard residential loan.
If its a "market value" comp, then you can still be compared to homes with less acreage, if those are readily available, but you would get an adjustment for your additional land.
There's a principal of value in real estate that the "buildable lot" has a certain value - whether thats 1/6 of an acre or 2 acres, or 5 acres - and then the remaining land has lesser value.
For example, in pricing an estate lot subdivision with a minimum acreage requirement of 1.5 acres per lot, the 1.5 acre lots have the highest value per acre and the 5 acre lots have a lower average price per acre.