Maybe this will help explain it
Your original repayment mortgage agreement was that you paid back interest and capital every year.
Assuming that it was close to the beginning of the mortgage, the figures would be something like this
I have made up the figures purely for illustration.
Jan 2009 Outstanding amount on mortgage 220K
Monthly mortgage €1000. Of the €12000 you pay in the year, 9000 pays off the interest, and 3000 off the capital, so at the end of the year , the outstanding mortgage is 217K. The next year, you pay 12000, but the interest is only 8500, so you pay 3500 off the capital, and so on for 30 years.
Over the course of the year,
In month 1, you are paying interest on 220,000
In month 2, you are paying interest on 2219250
In month 3, you are paying interest on 218500 etc
But, you have changed this to interest only, so you are paying interest on 220,000 for the whole year, so the interest will be higher.
You owe 220K, and this in not going down, so you pay interest on the whole amount of 220K, until you start paying off the capital again.
For as long as you are not paying the capital, you are paying interest on the whole capital.