They key is "not efficiently enough". The body volume (where heat is produced) is propoprtional to the cube of linear dimensions, when surface area is proportio

They key is "not efficiently enough". The body volume (where heat is produced) is propoprtional to the cube of linear dimensions, when surface area is proportional only to the square — hence the bigger you are the harder it is to get read of extra heat. And vice versa: the smaller you are the more effort it takes to keep yourself warm.