Cosmos blooming early

No - they should grow taller than that - however, you have a choice. You could try pinching out the growing tips to try to force the plants into growing bushier - that won't make them taller, though, I'm afraid! Or - just enjoy them as they are!

I really have no idea why they should behave like this - unless you bought a dwarf species? Or your weather was different from its usual pattern?

I grow Cosmos every year, last summer I planted some in my garden and some in my fathers garden, mine were small whereas his grew over 3ft. in height, all came from the same seedstock however they all flowered for weeks,I think you just have to accept they are like us Rolf and come in all sizes ...........