If your equipment is designed so it always leaves a liquid in the mash tun or kettle (lets say a quart). That quart is 1/20th of 5 gallons so it will contain 1/20th of the fermentables from the wort. If you switched to a 2.5 gallon batch, that quart could be 1/10th of the fermentables. You can make up for this by increasing the grain bill.
It mostly applies to kettles with a ball valve where you can't drain all the liquid. Less so to batch sparging in a mash tun, not really at all to fly sparging.