Once you have bought your smallcase, and want to increase your exposure to it - you can invest more in your smallcase
From your Investments page, click on the smallcase you want to invest more in and click the Invest More button on the right.
The Invest More page helps you buy more of what you currently hold, meaning it will have a minimum amount to maintain the weighting scheme. You can increase this amount to invest whatever amount you like, and it will be adjusted to the closest amount required to maintain the weights.
You can use this functionality to double down on your investments anytime you like. You can also invest more through SIP.