What is the difference between these two phrases as they both appear to mean "Thank you very much".
Looking at Google Ngram I see that большое спасибо is more common. Is there any difference or one simply a more formal way of expressing this.
Is the difference "A big thank you" versus "Many thanks"?
The common order in Russian is adjective precedes whatever it used for, so that's the reason you've figured out that "большое спасибо" is overwhelmingly more popular. Well, it's slightly more complicated than that, because Russian counterpart for English phrases in form "noun + is + adjective", like "the world is big" comes like "noun + adjective" like "мир большой", but this has nothing to do with you particular case, so let's just mention this fact but ignore it further.
So "большое спасибо" is indeed the most used form, however Russian is not that strict about order and in particular cases, in casual usage, the order can just be swapped, like in phrase:
Вчера в магазине видела такое платье красивое, но так и не решилась его купить.
or
Спасибо тебе большое, Пётр.
I'm not aware of any semantical differences or additional connotations that are exclusively specific for "спасибо большое". Both, for instance, can be say ironically or, say, passive-aggressively. Both can be a form of sincere gratitude.
The meaning is the same ('Thank you very much'). The difference is mainly stylistic: "Спасибо большое" is more informal and "conversational" while "Большое спасибо" is more formal and purposeful.
Mind you, Google Ngram searches over books so no wonder that "большое спасибо" prevails. In everyday conversations you are more likely to hear "спасибо большое".
It's the same. The Russians can change order of words in their speech just like in Latin. Normally you would say "большое спасибо" but you don't have to. You can just say "спасибо". It's OK. But if you have already said "спасибо" and feel like it is not enough, you can add in the end "большое" to sound nicer.
