They look to be normal for a catamaran. The keel depth is 3.5' and the rudders are just shy of the keel, so I would put the rudder length at ~24". This is the same as ours. In addition, they spent some time making them an efficient shape rather than just two planks.

There is no "usual excuse" for catamaran rudders - do you expect them to be 5' deep on a 3.5' draft boat? What you think are undersized rudders based on monohull experience are the norm for catamarans, and they work quite well at all speeds. Remember, there are two of them and they are spaced 20' apart with each one 10' off the centerline. The hull prismatic, hull rocker, lack of heel and constant CE/CG are other aspects that make catamaran steering systems different from what is normal for monohulls.

Catamarans are different from monohulls in several areas, and one must understand those areas in making comparisons and judgements.

Mark