The reason Lyndell's codes are short is because they are in one of the special internal Pronto formats as opposed to learned format. Pronto set up special formats for the most common codes, one of them being the NEC code which is declared by the prefix of 900x. The advantage of those (internal) formats is that they are precise with regard to timing as opposed to the learned format which is susceptable to noise during the learning process, and are very efficient storage wise..

There is at least one technical paper in the forum files that describes the internal formats of the Pronto, and while a bit complicated, makes for worthwhile reading if you want to understand how the Pronto generates it's IR timing patterns.