So, for some reason, the amount of rocket parts that a silo can take is 2^32 - 1 (or, 4,294,967,295). So, if you paid for the whole API - why not use the whole API?
However, because making 43 billion blue circuits, LDS and rocket fuel is kind of a lot - there is a mod setting to give the rocket silo 16 module slots instead of the initial 4.