Miguy had it right. You will get better hop utilization from doing full boils. Unless your doing extract batches you will generally have more than one gallon to boil anyway as you will mash grains with 1.25-1.5quarts of water per pound of grain. Most recipes are more than 3-4pounds of grain for a typical 5gallon batch. So at the very least you have a little more than a gallon just from the mash but then there is sparge water to rinse the grains plus taking into account your boil off rate usually about a gallon an hour so at minimum I'd need at least a 2gallon boil size to wind up with a gallon of wort after boiling. I usually get 6.75gallons boiled to 5.5. That gives me a full 5 into the keg.
Other than hop utilization your gravity will also be affected if you only boil a minimal amount and top off your also diluting your ABV content. IE if you had a 1.050 wort at 1 gallon and topped off to 5 gallons you'd basically have a 5 gallon 1.010 wort.
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