Schottky Defect in AB2 type crystals
Schottky defect occurs when equal number of positive and negative ions are missing from the lattice thereby maintaining net electrical neautrality and leading to a decreased density in the lattice (since there are now less number of ions) .
It is observed to be occurring in cations and anions of similar sizes. 

NOW CONSIDER THAT AN ION IS AT THE LATTICE POSITION AND A DEFECT IS TO BE OCCURRED.

There are 2 ways stoichiometric conditions can be satisfied. - either by loosing this ion as well as another of opposite charge in equal ratio(SCHOTTKY) ,
OR not losing this ion at all , but just shifting it's position from lattice position to interstitial one.(FRENKEL)----
If we think logically ,

Since it's a stoichiometric defect it should maintain neautrality as well as loose cations and anions in a certain ratio. 

If the sizes were very different , there are chances the smaller ion would occupy an interstitial space causing it to create a hole and still maintain neutrality and ratio without losing any ions. (Which is what happens in Frenkel defect) . Since however ions are of similar sizes , one can't occupy an interstitial space .. and therefore losing equal no. Of anions and cations is the only way to go. 
You can think of these as 2 cases of defect... Yes it's not necessary the ions are of similar sizes for a defect to occur BUT IF THEY ARE...They are most likely to cause the Schottky defect and if not , then Frenkel Defect. 
As for your NaCl doubt , I would suggest try finding the critical radius ratio for one ion to occupy the interstitial space . This way you can verify whether it can perform Frenkel defect or not ..Or why it's performing Schottky. 
Hope this helps.

EDIT:- I can't find any examples of ${\ce{AB2}} $ type as such , but you can again try finding critical ratio of ions in ${\ce{AB2}} $ lattices to decide what type of defect they are likely to occur.