If it says "Add 3lb of honey to 1gal of water", I'd start with a gallon of water, then add 3lbs of honey to it. This implies that you'll need a container larger than 1 gallon.
Even in making my simple syrups for back-sweetening or adjusting the starting wine must the results are normally pretty close to - 1 cup water plus 2 cups sugar equals about 2 cups simple syrup.
So for Meads or combinations with disolveable item, the result will be something less than 1 + 1 = 2 and more than 1 + 1 = 1 Another case of "It depends."