It was definitely the zoo where I saw them, and I think the big Clown Loaches were really chunky and dark coloured.
I think chunky and coloured is what you want to see in captivity. Good colour and lack of bony appearance certainly indicates a healthy clown loach. Any patches, spots, etc., pinched/bony appears is either a water quality issue or disease. I wouldn't consider wild caught clown loaches very healthy when arriving to a life in captivity....Although a lot of captive raised are too chunky...
I read another article last night. It was in some Asian, local to clown loaches habitat language,so I used google translate. It stated the same, majority of clown loaches caught are average sized, largest up to 30cm(12 iches) total length.
A big clown loach between 10-12 inch costs hundreds when exproted, so they'd be sorting those type of clowns before exporting them out of the country(perhaps against local laws) for the "big bucks". But it doesn't mean what they catch is mostly that size as those sizes are rather rare to either purchase, or to raise.
Most people that have kept them for years know not all of them grow as big as "advertised" on the net...And the same seems to be the story in all reports from the wild, rarely reach 12 inch.
The loaches in the zoo would have been purchased by size, donated loaches, wild caught old loaches, etc... I highly doubt it they raised them themselves.....So is the case in many people's tanks who were hunting for large loaches to make a sizeable school. But if you raise them yourself, you face the domino/pyramid effect. Some large, some half the size of their siblings/cousins the exact same age.