If I have a tap flowing into a bucket, then once the bucket is full the amount of water flowing over the top will exactly equal the amount of water flowing in from the tap.

If I increase the size of the bucket, the the amount of water flowing over the top will also exactly equal the amount flowing from the tap.

There's also scale to consider: global warming is more like if my bucket was very soft, and I've stretched the plastic a little bit to give it slightly more volume (the analogy breaks down beyond this point).