It's not incorrect. Most dictionaries would say that big it's not just for size, but also used for something of great importance or major concern, like en.oxforddictionaries.com says:
Of considerable importance or seriousness.
‘it's a big decision’
‘his biggest problem is money’
‘he made a big mistake’
Also, it's easily to understand, then even if it was technically incorrect it would be a good use in a informational conversation. People often use big to measure other things that are not physical and doesn't have a "size", like "that guy is a big fan" not meaning the guy is big, but that he has great appreciation for something/someone or like "big eater" that would imply that the person eats a lot, and not that the person has a big size.
That said, although it's correct, you could use other words that would be more commonly used in that sentence, like: favorite or main.