Made silos have double the capacity, cost more. 

This is combined with slightly reducing the amount of tiberium stored in Refineries. Part of the reason for this is that refineries are targeted lots by ion cannons (one-shot kill) and nukes. Whenever this happens, you lose the money that's in them. By making a silo that stores more, you can store the tib farther away and keep a bit more of it safe.

The other part of the reason is to bring silos into the game as a somewhat strategic structure. By having more money, they are more capturable, because you can steal what's in them. It might also make sense to maybe airstrike them so that the opponent loses what money is in them.
The way to counteract this problem is to build more silos. They cost a bit more though (double) so that you have to think about how many you want.