Kanji for dog

So I am a Japanese I student, and I have been learning kanji because my class goes at a very slow and boring pace. I found the kanji for dog, 犬, and was reading about how combining the parts of a kanji can help you determine its meaning. Why do the two parts of this kanji mean big and spot? Is there a reason behind this or is it just one of those nonsensical things like English spelling?

by TheSwordMaster22