You may want to look into making the Y-axis logarithmic.
Although, using logarithmic axis may not be very intuitive. This explains the use of zero-based versus log axis very well: Should you use a log scale vs. zero-based scale for comparisons?
Hope this helps!
Well the values are the values! so without distorting things there's no too much you can do. A couple of things might help....
One would be to switch the order (you can either sort the stacking dimension, or drag and drop to change the order in the legend) so your small values appear at the top. You could then make the blue slightly lighter (say from the Tableau Medium pallet), and then use the Tableau pallet for the Orange and Green.
Hope this helps.