The interest you pay initially is higher since the principal balance is higher.  As you reduce your balance interest is reduced, and more goes to principal.

Good Luck and hope this helps.

If you pay $1 extra in month #1, you've cut off the interest expense for 359 payments for that $1.

if you pay the same $1 extra in principal at month #100, then you've only cut off the interest expense

for that $1 for 260 months.

Steve

One other way to think of why extra payments early on have a greater impact is to realize that when you pay additional money on your mortgage, it is like investing money that returns you the rate of interest you have on your mortgage.  We have all heard that it is better to invest for retirement when we are younger then it is when we are older.  Rather than seeing you balance increase more over time, as you would when you invest in your retirement account; you will see your debt (mortgage balance) decrease to a greater extent over time if you apply extra payments in the early years of your mortgage.