That's a misconception. Earth is still pretty much in an equilibrium and emits as much as it gets from the sun. Global warming is due to the heat staying a bit longer in the system, not due to emitting less.
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).