It also seems large for its type. Compare Beretta Pico @ 5.1” & 11.5oz.
Not vs. 9mm as this is not an “or” question but an “ and” one :)
It's supposed to be large for its type. A heavier slide allows the use of a lighter recoil spring, which in turn gives the Shield EZ a much easier to manipulate slide. The larger frame and slide weight, coupled with the weak .380 also provides for the reduced recoil.
Keep in mind the target audience for this gun: New shooters, women, and senior citizens.