There are some chapters that are small because they haze, or they're all engaged in a variety of questionable activities, or they have no desire to do anything. If those die out, well, groovy-poo.
But there are times when the rushees come back from a party and say "I really liked XYZ, they seemed like nice girls and they all love their sorority so much. But I don't want to join there. They are too small and they have a reputation as the "unpopular" house."
I will paraphrase I believe KappaKittyCat, and say FU#$ING HELLO!! If you all join the small chapter, it won't BE small anymore!! If you are so friggin popular
, join the unpopular house and change it.
Now I don't expect a bunch of freshmen women who don't know each other to get together like one of those Mickey Rooney/Judy Garland movies (hey kids, let's revitalize a sorority!). This is where the Panhellenic and Greek advisor should come in. Don't try to get the women to join by stealth means like not letting them drop the smaller group. That just makes people mad. Get the unbidded women together after recruitment and ask if they are interested in being part of a block class. It's easier to go out on a limb when you know there will be other people with you. Not only that, let campus leaders who maybe didn't go through rush know about this opportunity. Lots of women would love being Greek but they hate rush.
Oh, and I understand the concept of only wanting one sorority. I would never say go where you don't feel comfy. But if the only reason you want Gamma Phi is because they hang out with the Pi Kapps, or you want Theta Phi Alpha because they have the homecoming queen, well, you deserve to end up bidless...shallow beeyotch.
Edited to add partly in response to s & s's post, a lot of it also depends on the attitude of the sorority's national headquarters. If they are happy with the direction of the chapter, find positive things about them, don't mind that it is slightly smaller and accept that the group might never be on top that can go a long way. If the sisters feel that positivity from their higher-ups, they will be able to translate it into rushing. However, if they're doing nothing but hassling the sisters to improve numbers at any cost - even if it makes all the sisters miserable and desperate - they probably won't be able to crawl out of the hole. It's hard to be positive and upbeat in rush and tell women how wonderful your sorority experience is when you (metaphorically) have a gun at your back.