I can think of two ways
1. Manually move the axes position
>> ah = subplot(2,2,1)
>> currentPos = get(ah,'Position');
>> set(ah,'Position',currentPos - [10,10,0,0])
2. Make the axes span multiple subplots of a finer grid. For instance, try this:
>> figure(1); subplot(4,4,[1 2 5 6])
>> figure(1); subplot(4,4,[3 4 7 8])
>> figure(2); subplot(2,2,1)
>> figure(2); subplot(2,2,2)
You will see that figure 1 has tighter spacing
