Why have the bottom leaves turned yellow and shriv...

Organoman has given you a decent and correct answer. It's time to pot up to a bigger pot with fresh soil. In future don't use them cardboard pot things, they absorb most the moisture out the soil, it takes ages for the roots to break through it also. You would be better off starting in that seedling tray and after 5 days transplant them to the final pot.

Ok then. How come the ones in the smaller spaces in the tray are growing better than the bigger cups ?

Because they are in tiny cups that are totally inadequate for the plants of that size. The soil will be so depleted that there is absolutely no goodness left and the tiny root system in that small amount of dirt won't help matters either. Basically, you can't grow an elephant in a canary cage. Blunt answer, but there is no sugar coating this situation.

I'd say they have out grown their space and used up the available nutrients. Nothing to worry about Growmie. Transplant into final containers and all should be grand ✌️