Originally Posted by muse435
Since i have never done this, what do you mean?
There are systemic losses that occur throughout the brewing process. For example, you lose some of the wort due to hot & cold break, from wort that remains in tubing or in the bottom of your brew kettle, from beer that stays in the primary fermenter when racking to another fermenter or kegging/bottling, or from absorption from dry hopping or adding fruit or other stuff.
For some of these, the amounts tend to be roughly the same no matter what size batch you do, although that is not always the case. For that reason, simply scaling your recipe by 1/5 could leave you less than 1 gallon even if you end up with 5 gallons normally.
Also, as a result scaling can also lead you wrong on hops additions, because the preboil gravity might be different than it would be for a larger batch, which changes the amount of alpha acids that get isomerized during the boil, affecting bitterness.