The fact that mobiles are slim means that the components used to make it are very compressed. It means that the processor and all other things that make up the phone are very small. The processors are small and have to process a lot of heavy applications and they therefore heat up a lot. From my perspective, it is because the components are compressed inside. they are close and they therefore transfer heat to each other. Have you noticed how laptops heat up faster tha computers It is because they do not have enough space to cool down. Slim phones barely have space to let out some steam. Curent pases through them a lot and they carry out complex activities that generate a lot of heat.