Why does my iPhone get hot with this app only?

I have an app called TotSpot which I use to sell my kids' clothing. I've had it for about two or three months and I finally realized that it is THIS app that makes my phone get very hot very fast. I have an iPhone 4S by the way. Anyway, as soon as I open the app, my phone gradually but yet quickly gets hot to the point I can't hold it to browse for a long time. As soon as I close the app, my phone cools down back to normal. What gives? What's going on? Yes, this app also drains my battery fast… But why?

This app uses more memory than the other. That means its processing take more time resulting in hotness of the phone body.