problem with showing a big matrix

i have this program but as my matrix get bigger (values up to 100) isosurface just shows me an empty cube...what should i do to show it right?

close all
clear all
nx=100;
ny=100;
nz=100;
mat=zeros(nx,ny,nz);
mat(30:70,30:70,30:70)=0.3;
mat(48:53,48:53,30:70)=0;
isosurface(mat);
xlim([1 nx]);
ylim([1 ny]);
zlim([1 nz]);

[changiz]

when turning nz=100 to nz=98 it shows right!!!!!!!!!!!