Right now, your meshgird command is attempting to create 2 60000x60000 matrices... which are going to require around 26 GB each. However, luckily, that's not actually what you want to do.
Is your real data actually completely scattered, as in this example? Or does it already lie on a grid, but is simply stored in vectors?
If it's truly scattered:
F = scatteredInterpolant(x,y,z);
[x,y] = meshgrid(linspace(min(a),max(a),100), linspace(min(b),max(b),100));
z = F(x,y);
If it lies on a grid already, you can eliminate the triagulation and simply rearrange your data into the appropriate grids.