Small things can have large effects.
Skeptics often times tell us that Carbon Dioxide levels are too small to have any effect on the climate. The assumption is that small amounts of anything cannot have a meaningful impact. Common sense proves that assumption wrong since there are many substances that can affect you even in small amounts.
Imagine that a doctor prescribes some medication and you are supposed to take one capsule a day. That one capsule is very small in size when compared to your body but its effect on your body is what matters, not its size.
Now imagine that you decide to take three or even four times the medicine that the doctor prescribed thinking that you can get well three times faster. Instead of getting better you will overdose on that medication.
Greenhouse gasses, like CO2, are no different. The design of the molecule, not how much of it there is in the atmosphere, is what gives it a much greater ability to retain heat than other gasses.