With straight line repayment, the interest on the loan is calculated equally per period and the Equated Monthly Installment (EMI is the same. The same component of interest is payable for each period.
The declining balance method calculates interest at periodic intervals on the amount of the principal not yet repaid. While the repayment amounts, EMI are equal, the interest component of the EMI is larger in the initial repayments and gradually reduces over time when compared to the principal amount.
The exact percentage allocated towards payment of the principal depends on the interest rate. Even though periodic EMI repayments amounts do not change, the proportion of principal and interest components changes with time. With each successive repayment, more is allocated towards the principal and less towards interest.