My guppies at work are in a small bowl (probably about 20L) and are going great guns(started with 4, now there's about 40 fry also), I attribute this to a couple of things:
Stable temp - because its at work in air con it sits at around 24 constantly
Feeding - I feed them twice a day enough that its gone in 2mins, but they have two fasting days (I don't come in on the weekend)
Hands out - I never fiddle in this tank, they are lucky to get a water change of 30% once in 3 weeks, I am careful about adding new water though, I do it slowly over a few hours (a cup here and there).
Of course this is what works for me and won't work for everyone, I do believe in trying to not change things too much. Every time you put hands in tanks it's a risk you will have chemicals on them.