I saw on this website http://www.secretofchallah.com/50708/When-to-separate-challah#opinions that one should not separate challah if using less than 8.5 cups or about 2 lbs. I know there are various opinions on how much it takes to separate with a bracha, but I've never heard of a minimum for separating without a bracha. Any ideas of why this would be?
The four opinions on the linked website differ on the exact standard of volume measurements used by halacha.
Each opinion holds the minimum amount of flour that necessitates separating chala, which is a mitzva and is accompanied by a bracha, is the volume of 43.2 Eggs. Any less than that and the requirement of chala does not apply. (324:1)
The Egg is a standard unit of volume in halacha whose exact size is a matter of debate. The larger of the two numbers listed for each opinion is that person's precise estimation of 43.2 Eggs. The smaller of the numbers is 43.2 Eggs according to another (non-preferred) method of calculation. These opinions hold that one should act stringently by performing the mitzva for the lesser calculations as well, but should not make a bracha because we do not truly hold that the minimum required volume has been reached.
See Rabbi Josh Flug's extensive detailed explanation of the topic here.
According to Isaac Moses' understanding of the question, the following might be a reason not to separate challah when the dough is below the minimum size:
The halachah is that even after baking, loaves can combine for purposes of challah if they're all in the same container (Yoreh De'ah 325:1). So suppose that a person makes two batches of dough, each 1-1/2 pounds, separates challah from each one, and then bakes each batch into a separate loaf of bread. So far, though, challah has not been validly taken from either batch, because 1-1/2 pounds is below the minimum size for the mitzvah of challah. Well, now, if both loaves are placed into one breadbox, they become obligated to have challah taken from them after all! But the baker won't realize that, since they think that they have already separated challah correctly.