Why is this pointer null

The size limit of an object is implementation defined, per Annex B of the standard [1]. Your struct is of an absurd size.

If the struct is:

struct Foo
{
    char arr1[INT_MAX];
    //char arr2[INT_MAX];
    char ch1;
    char ch2;
};

… the size of your struct in a relatively recent version of 64-bit MSVC appears to be around 2147483649 bytes. If you then add in arr2, suddenly sizeof will tell you that Foo is of size 1.

The C++ standard (Annex B) states that the compiler must document limitations, which MSVC does [2]. It states that it follows the recommended limit. Annex B, Section 2.17 provides a recommended limit of 262144(?) for the size of an object. While it’s clear that MSVC can handle more than that, it documents that it follows that minimum recommendation so I’d assume you should take care when your object size is more than that.

[1] http://eel.is/c++draft/implimits

[2] https://learn.microsoft.com/en-us/cpp/cpp/compiler-limits?view=vs-2019