Less likely, yes. Exponentially less, no. You'd be roughly right if molecule formation was just a matter of random chance associations, which is true in the diffuse ISM, but it is not true in dense clouds where molecules form. A large fraction of all molecules form in clouds that get cold enough that the molecules stick to the surfaces of solid (dust) particles. Once they're there, they're in rich company: there's hydrogen, carbon, oxygen, etc. in abundance - and then more "normal" chemical processes (i.e., things you might find happening on Earth) start to take over. So yes, the numbers game starts to reverse pretty hard!

Purely from gas-phase processes, though, you're basically right; we expect that most molecules with >5 atoms rarely form in the gas phase. We usually draw the line at methanol, CH_3OH, which is a bottleneck in the formation of more complex molecules.