People who say спасибо большое first want to say thank you and after they have pronounced спасибо they want to say thank you very much. This happens when we speak, we speak faster than we think. When we write we have more time to think and therefore use the right word order - большое спасибо.

Remember, when you speak Russian it is not important which word you say first and which word you say second.

To be honest there are not any difference in meanings. This phrases are equal. The order of words dependes only on personal speech style