Please refer to mortgage amortization schedule. My reference was for first payment only. You pay interest only for the principal amount you owe. If you load up and pay down your principal then your amortization will change and interest rate will change for your benefit accordingly.

Google:- mortgage amortization schedule > click on bankrate.com/mortgage/amortization and fill out your number and check.

You will understand my math and language. Interest you pay is directly related to principal balance of your loan. And again, you are right as well since your principal is high at the beginning you pay more interest and at/around end your principal is low so you pay less interest.

Jai Nepal