I'll throw in my own two cents and say that the reason for why the British Empire was so big was because the largest territories, Canada( currently the second largest nation) and Australia( current the sixth largest), were rather simple to take since they were sparsely populated and had various disadvantages against the British. ( weak weapons and succumbing to disease) Otherwise, according to my calculations, the British Empire would be the third largest Empire in history, with the Mongolian Empire and Russian empire coming in first and second, respectively. Though one could make the same arguments for those empires, which ruled sparsely populated territory as well.