I'll admit that my answer was completely wrong, so I won't even bother repeating it, but as I tried to form some sentences of my own to become more familiar with this grammatical structure, I began to wonder if this sentence is correct as is or if it should include "daha" before "büyük." I especially began to wonder about this after going to tatoeba.org and seeing virtually every example of a comparative structure (such as this one) include "daha" before the adjective. Then, a little further in this lesson, I read a post that cleared it all up for me. You can link to it here: https://www.duolingo.com/comment/8756028. If you don't want to link to it, the information at the link asserts that the "daha" is optional.
You can report it. (We just had a big debate in another thread about whether "than me" should be an accepted/preferred translation for comparative statements. "Than me" is so widely used that I'm in favor of using it as the preferred translation, but I imagine they'll accept "than I" as an alternative if you report it.)