"He needs a big investment."
Translation:Il a besoin d'un gros investissement.
There are expressions in every language where certain word just fit naturally and others don't. With financial investments, the word is "gros", not "grand". Sitesurf is trying to explain that even though a francophone would understand you, it is not the way they would say it. So let's go with natural French, not unnatural French, i.e., un gros investissement.
I understand for this sentence, but in general I admit I don't know when to use "gros" instead of "grand." I've also noticed in other contexts like with a "gros cochon," but I still haven't figured out how/why that's different from a "grand cochon." Is it just a question of memorizing the words that take "gros" instead of "grand"?