I'm not sure how your two statements here are equivalent?
a = sparse(1e6, 1e6);
creates a sparse matrix of all zeros of dimensions (1e6, 1e6), making only 1e12 elements, and nothing needs to be allocated since there are no non-zero values
b = accumarray([1 1], 1, [2^31 1], @sum, , true);
Creates a sparse vector of size (2^31, 1) with a single nonzero value allocated at (1,1). The equivalent call to this is surely:
>> a = sparse(1, 1, 1, 2^31, 1)
which fails with:
??? Error using ==> sparse Sparse matrix sizes must be non-negative integers less than MAXSIZE as defined by COMPUTER. Use HELP COMPUTER for more details.
Which is probably the same error in another guise.
in fact even
a = sparse(2^31, 1)
fails with this error.