Before an abscess has a collection of inflammatory debris to drain, it is a solid mass.
The transition is from cellulitis (infected, inflamed soft tissue) to an actual collection of pus cells in a pocket–an abscess.
As USCDiver notes, you have to have “pus” before you can drain it.
If you incise an absess too early, all you do is end up making a hole in solid, inflamed tissue and nothing is accomplished.
We do not want an abscess to “get as big as possible.” However waiting when all that is present is inflamed soft tissue will determine if an abscess is going to form at all, and will sometimes localize more clearly where the best incision point is to drain it. Also, as an abscess forms it’s not uncommon for the surrounding tissue to get less inflamed so that you end up with one specific area inside of which is the abscess, instead of a diffusely reddened larger area with no localization.