Calling Sandy a "weak storm" is very misleading. It is true that Sandy was only a Category 2 storm at its peak and Category 1 storm at landfall. However, those levels are based only on peak wind speed.

In terms of total storm energy there is a classification known as "Integreated Kinetic Intensity" (IKE) which is a better measure of the total power of the storm. IKE takes into account both the wind speed and the area that the storm covers. Measured that way Sandy was more powerful than Katrina.

See weather researcher Brian McNoldy's comments at Superstorm Sandy packed more total energy than Hurricane Katrina at landfall .
Some other comments on Sandy by McNoldy are at Hurricane Researcher Brian McNoldy on the Science Behind Sandy.

Finally note that how destructive a storm surge is depends a lot on the land elevations near the coast line. Much of the Gulf Coast tends to be relatively low elevation for a considerable distance inland. This means a storm surge can penetrate much further inland and do much more damage.