The second largest share is given to the Current Military, with 28% of the budget, while the Past Military receive 10% less.
"It" isn't required, nor are several other words in your original example, as you can see above. In your sentence, "it" simply referred to the budget, not to the percentage.
There is a budget.
The Current Military gets 28% of it.
The Past Military gets 18% of it.