The DNA amount you receive when you care for a creature depends on a number of factors:

• The preference of the creature. Some creatures prefer eating, some prefer playing, some prefer being interacted with. Experiment to discover their preference!
• The rarity of the creature: Common creatures give more DNA than rare creatures. Rare creatures give more DNA than Epic creatures.
• The needs of the creature. For example, a hungry creature will give more DNA than if you already fed it recently.

Use the preview tooltip (pressing and holding the care button) to see how much DNA you will get, and try different combinations!