Why would I need a bigger one?
With a bigger brooder, there can be a warm part and a cool part. That way the chicks can move back and forth to find the temperature they like best.
And when they have access to a cool area, they start getting used to cooler temperatures from a young age, but without being stressed about it (because they can always go warm up again when they want to.) So it can help them be ready to move outside sooner.
Also, with a bigger brooder, they have more room to move around, exercising and playing, and they are less likely to pick on each other (boredom) and step on each other (bad for the small chicks in the group.) Yours pretty much have room to eat, drink, and sleep, but not much else. They can't scratch around foraging, or take a dust bath, without bumping into each other or the sides of the bin or the food and water, and they pobably kick lots of bedding into the food and water.
A guideline I've often seen is 1/2 square foot of space per chick for the first two weeks, then 1 square foot per chick until age 4 weeks, then 2 square feet per chick until age 8 weeks, and after 8 weeks they should have 4 square feet per bird, just like adult chickens should.
So at 3.5 weeks, 6 chicks would need 6 square feet of space, which is 2 feet by 3 feet--I don't think your brooder is anywhere close to that big. And in another week or so, it should be sized up to 2 square feet each, which would be 3 by 4 feet for your 6 chicks.