I have not needed to make such a pot but I just did to test this out, it does in fact appear in game as weighing 600 grams not the expected 12 kg. To be sure it is not metric conversion I checked it in imperial and it is only 1.4 pounds.
The script seems to be correct, just not working. I changed the weight from 27 to 2,7 and it still makes a pot weighing 1, 4 pounds. It seems the [weight:] parameter does not accept decimals and it does not accept numbers greater than 9, any number with 2 or more digits is the same weight, I tried several weight settings, below 9 seems to work as expected, above 10 everything weighs 1.4 kg but a some point the weight drops to 600g. This assumption is wrong as other recipes use decimals and barrels and kegs weights work correctly. I need to check them again as it could be a change in the base code is affecting all mod weights.
Just checked the tub mod, it is listed as weighing 16 and is made in game weighing 7.3 empty, so not a game mechanic problem but an error in the earthworking script. I thought it might be due to it not having the line [TYPE:container] but adding that does not help. I have totally rewritten the code, to avoid cut paste errors with none printing characters. I have put it in a different mod section to avoid something in earthenware affecting it, it still does not work but I cannot see why.