MATLAB does not copy the matrix if you call it as parameter, or even assign it to new variable
BIG = rand(1e4);
B = BIG;
C = foo(BIG);
function C = foo(A)
C = A;
end
A, B, C use a shared-memory of data.
So remove this global and program normally as you would and let MATLAB does the work.
NOTE: However if you do this there is 2 big matrices in memory after the third statement
BIG = rand(1e4);
B = BIG;
B(1) = 10;
