How to reduce size of the app

Hello,

I'm developing an application for iphone with Unity. I use textures for 3D object and in the unity folder, the size of all textures is less than 1Mo. However, when I build the application with XCode, I have an app that size 28 Mo. I tried to remove all textures and the size of the app is now 15 Mo. Does anyone know how to reduce the size of textures in the iphone app?

Thanks

Marc