Not difficult.....John was the precursor, the announcer of Jesus, the messiah. So, he was a BIG DEAL in the fulfillment of messianic prophecy. As great as he was, though, he was still simply gonna be another recipient of salvation and eternal reward. If you notice the context of Matthew, there are those that approach Jesus, sent by John. John was a BIG DEAL to them. He was their teacher, much in the same way as Jesus had his followers. So, they ask him a question and that question originated with John, in other words, JOHN wanted to know, "Are you the one who is to come, or should we expect another?"(Matt 11:3) You see, as much as John knew about Jesus, he wasn't sure...
Jesus, then, in order to solildfy his Messiahship and help those followers of John, said what he said in Matthew 11:11. In essence Jesus is saying, "John, as great as he is to you guys and in the fulfillment of my messiahship, he is still no greater than any one of you who accept the kingdom. In fact, he will be the least...."
As much as the ORG tries to paint the picture that he will be part of those that get resurrected to earth and thus lower than anyone in heaven, the New Testament does not speak of any such thing. If you are a Christian there is one hope....go to heaven that's it.