Try pinching out the flower buds as soon as they appear & moving them to a cool place to bulk up before planting out. It's common for cosmos to flower too soon once they warm up. It's not too late to sow more seed.
You'll still have all the energy going into flowering even in bigger pots if they're still in the sun though

That's amazing Darren. It never occurred to me to pinch them out but now you suggest it. I can see it makes sense. Thankyou for that enlightening post.

I've been pinching out flower buds on some of my toms which sort of feels wrong but there doesn't seem to be any adverse effects.
Cosmos flower for a long time so even if you do get the odd early bloomer, given more root space in the ground & a feed they should still spread nicely

Hi Darren, just wondering why you're pinching out the flower buds on your toms ?, the sooner they flower, the sooner you get fruit, Derek.

They're beefsteak ones so I want the plants to bulk up more before they fruit Derek. It's just a balancing act to make the plant grow as strongly as possible & build roots that'll carry a good crop in a month or two from now.
Well, that's my thinking, I may be wrong?

Hi. Darren, you're probably right, if they're going to bear heavy fruits, they probably need to be quite a strong plant, never grown beefsteak toms, so don't know a lot about them, when I've grown ''ordinary toms'' I've always let them flower as soon as possible to get the fruit, Derek

It's not just the weight, it's building the plant up so it can absorb & send as much nutrition to the fruit as possible in our short season. I'm just manipulating things for an optimal crop, touch wood 😊

As for Cosmos, it usually only blooms that small if it is suffering from drought, or starving for nitrogen.