It surprises me that a free addon airport could be so big since I've got many payware airports that would seem to be much bigger and I've never encountered a problem.
The problem is not the total size, but the size of the AFCAD. Commercial developers knows very well that, lots of the issue/bugs in a scenery are usually related to the AFCAD, so they keep it separate in a small file, in order to be able to update it more quickly. AFCADs for the default scenery are all quite small, even if they contain more than one airport, so GSX tries to take advantage of this, in order to speed up the cache regeneration time, so it won't even look at a file, if it's considered "large".
However, the FSX SDK allows to place airport data (AFCAD) together with objects or even textures, and some freeware authors take advantage of this, making a single .BGL with all the scenery, because it might be easier to install by simply adding it to the "Addon scenery" folder, so they might exceed the default threshold we set, which is 1.2 MB.