What does this line mean?"Small Animals have a larger surface area relative to their volume they tend to loose body heat very fast when it is cold outside and they have to expend much energy to generate body heat through metabolism". but small animals have smaller surface area right?

Dear student,
Smaller animals have larger surface area is to volume ratio means that their surface area is relatively larger in comparison to volume therefore they tend to lose more heat when it is cold outside but larger animals have smaller surface area is to volume ratio as the volume is much greater than surface area. This can be understood by the simple mathematical calculation given below.


 In the table given above shows that as the side of the cube increases surface area and volume both increases but the ratio of surface area/ volume decreases. This shows that as the size of the organism increases, volume increases more in comparison to surface area therefore smaller organism have lager surface area than volume so surface area is to volume ratio is more. But in larger animal volume increases more in comparison to surface area therefore surface area is to volume ratio is smaller. 
Surface area is to volume ratio is more for smaller animal therefore they losses more heat to maintain body temperature when it is cold outside.

Regards