Welcome, and great looking fish!
I remember reading an article on 'Loaches OnLine' [IIRC] that recommended a 4' long tank minimum for young clowns up to ~5" and 6' length as the minimum after that. I always thought it was fairly good advice, and while I don't have any hard evidence I feel clowns are one of those species that definitely benefit from having a lot of space. I sold six clowns to a friend way back nearly 20 years ago, they were around 3-5" at the time and he had a couple of others around a similar size. They were kept in a 75g tank, then moved into a 30g for a couple of years while he was away studying, then back into the 75g after that. None of them ever got over about 6", and eventually they faded away and died. Maybe there were other factors at play, not enough water changes perhaps. I'm sure it is possible to grow big healthy clowns in a 4' tank with the right amount of filtration, water movement and water changes, but bigger is definitely better. Some fish (oscars for example) will still grow huge in small tanks with less than ideal conditions, others (like clowns) don't.
I'm kinda rambling sorry, and it's impossible to know for sure if the tank size for those early years had an effect on the size of your oldest loaches, but IMO it is possible. It's also possible it is just natural variation too. Male CLs are smaller than females, and any population will have an amount of natural variation. Regardless, they're great looking fish you have and the tank looks nice too! It's awesome to see people keeping them for such a long time.