This is a prime example of why the Church does not obligate anyone to believe in private revelations, or consider any message they may contain as a part of the "deposit of faith" -- those teachings which are integral to Catholic faith.
ANY revelation or supposed revelation by a saint or mystic regarding how many or what percentage of people are in hell, or exactly what sins put them there, is a private revelation only. The same goes for private revelations regarding purgatory and the sufferings of the Poor Souls.
If the Church approves such a revelation, or canonizes or beatifies the person who received it, that does NOT mean the revelation (or how it is interpreted by the visionary or others) is necessarily 100 percent accurate. It means, simply, that it is not directly contrary to the faith, and one is free to believe in or accept the revelation if one chooses. But you are also free to reject it if you find the revelation not helpful to your faith or piety.
Believing that only a small fraction of people make it into heaven may be helpful to someone's faith if, for example, it motivates them to repent of their sins, go to confession regularly, or increase their efforts to evangelize others. However, the same revelation can be very harmful if it drives someone to despair, or tempts them into spiritual pride because they imagine themselves to be part of an elite few who have achieved salvation, or relish the thought of all the people they dislike being damned for eternity :( .
I do remember that as a kid, probably around age 12 or 13, I got into some old Catholic books that my mom had bought years before, one of which was devoted to private revelations about purgatory. Well, that book was full of all sorts of frightening visions of people spending hundreds of years languishing in flames for venial sins like playing cards too much or dressing a little too fancily. (I'd really hate to think what the purgatorial penalty might be for spending too much time surfing the internet :eek:)
Now did this book motivate me to greater holiness? No, actually, it made me feel like "What's the use of even trying to be good, if I'm going to burn anyway after I die for every little sin I commit?" :shrug: Later on, however, I found some much more balanced and sensible material regarding the nature of purgatory, emphasizing that it is NOT a mini- or short-term version of hell, but far different, and even at its "worst," far better and closer to God than our earthly life.
Jesus did say that the gate to life was narrow and "few" entered into it, but he never got more specific than that. Note also that when his disciples began to despair, and ask "Then who can be saved?" Jesus emphasized that "with God, all things are possible."