Ha ha Sara 4 , did you by any chance add manure to the bed before planting the cosmos? The same thing happened to me last year. Hardly any flowers but I nearly had to take an axe to the stems in the autumn.
I'm still digging the stumps out...
I grew the cosmos seeds from G W magazine,in ordinary multi purpose ,in containers,several are well over three feet not as tall as yours but still beautful.
It's funny how so many flowering plants grow massive instead of flowering when they've got good soil. Greedy lot don't want to share the goodness with their offspring!
I grew the GW Cosmos Purity seeds and they are all around 4ft tall and flowering well now.
I grew GW Purity free seeds as well, mine are at least 3 foot tall, they even withstood the hottest of the weather with minimal watering.........goodness knows how tall they would have got with rain as well