As I've stated before, I don't believe this is because the brethren haven't been working as we should, I believe that the overall evangelistic effort done by the church has been powerful in the converting of the lost to Christ (while there are many individuals that do not "pull their weight," sadly) and while it might have something to do with our mindset about our confidence as I've discussed before, I'm not convinced overall. No, I believe the answer to her question is very simple, it came directly from the mouth of the Lord.
The answer is, plainly put, the majority of people will not go to heaven. If that is true, and it is (Matthew 7:21-23), then it stands to reason that the groups of Christians, those that follow the Christ (Acts 11:26), will be smaller than those groups that fail to do so because of one reason or another. The reason might be ignorance, which is never an excuse, or it might be haughtiness, but whatever the reason there will be people on the Judgment that believed they were working righteousness and yet were practicing lawlessness because of their failure to follow the Law of Christ (James 1:25).
"Not everyone who says to Me, 'Lord, Lord,' shall enter the kingdom of heaven, but he who does the will of My Father in heaven. Many will say to be in that day, 'Lord, Lord, have we not prophesied in Your name, cast out demons in Your name, and done many wonders in Your name? And then I will declare to them, 'I never knew you; depart from Me, you who practice lawlessness!" - Matthew 7:21-23
What does that mean for us? Should we feel discouraged that the church down the street has 15 gazillion people filling into the building on any given Sunday or that the church in the next city over boasts of their ability to reach "the lost," though those who are reaching them are lost as well? NO!
Let me speak to my brethren for a moment. Brethren, don't be discouraged that your congregation isn't huge. I'm a firm believer that smaller congregations are more beneficial to the edification and fellowship of the members thereof. While we might not have the numbers of the cash to do the great things that the denominations are able to do or even keep up with the larger congregations of the church, we should never feel discouraged. He is still with us, and as long as we stay with Him and continue to work for/with Him, nothing else matters.
Now, let me speak to those who might be viewing the churches of Christ with some amount of curiosity. Please don't allow the culture of megachurches to cloud your judgment when it comes to the Lord's body (cf. Ephesians 1:22-23). I know that churches of Christ are primarily small, but that's the way Christ designed it. The fact is, there are going to be millions, even billions, of people that Day that will confidently approach their judge only to be cast into torment because of their sins, one of which being a failure to follow His plan for the church. I hope you will not allow a society to decide whether or not you will attend, get to know and study with the local church of Christ in your area. We may be small, but that shouldn't make you turn us aside.
Remember, the Lord said that the few will enter into Heaven because of the difficulty of following Him (Matthew 7:13-14), but those that do are going to be rewarded beyond measure!