Dowels/straws that are about 1/4" wide are enough. Look at the ones Wilton sells at a store to understand the size.
How many dowels did you use and how did you measure how tall they needed to be?
A WASC cake is not really that heavy

To support a 9" tier you should use 5 to 7 dowels spread around an 8" circle. - none in the middle.