The denominator indicates the total number of parts in a fraction, not just the ones being considered. If a cake is cut into 8 pieces, 8 is the denominator even if you only eat 2 pieces.
Pro Tip 2/3
Relative Size
A larger denominator means smaller individual parts when the numerator is unchanged. 1/5 of a pizza is less than 1/3 of the same pizza because 5 is a bigger denominator.
Pro Tip 3/3
Not Just Quantity
Denominators can describe parts of a whole beyond physical items, like time or probability. In the probability of 1/52 of drawing an ace from a deck, 52 is the denominator.