It is a common myth to assume that because a koi is big it is therefore worth lots of money. Low quality inexpensive koi will get big just like their high quality cousins, that doesn't make them worth anything more then what you paid for them. As I said above, it's all about the quality of the koi, not necessarily the size. The reason you will see large koi sell for a lot of money is because the breeders almost never sell their best koi when they are small. They hold their best koi until they are bigger and worth more.
The way it works is like this. The breeder starts with say 100,000 koi from a single female. From that 80,000 never get sold, they are destroyed or turn into food. These usually have no color or they are deformed in some way. They take the 20,000 left and grow them for one season. At the end of the season, from those 20,000 they sell maybe 18,000 right away. They do this because they have determined that those koi will never be worth more when they are bigger. These are the koi that fill the $10, $30 and $50 tanks at the dealers shop. The remaining 2000 they keep because they feel they will be worth more next year. So the little koi you buy, they are from those first 20,000 meaning in the top 20% of the spawn, but were sold small because they lacked potential. That doesn't mean they are ugly, far from it. There are many beautiful little koi that will make wonderful pond pets. But from a financial point of view, these koi are not worth growing another year at the breeders. Growing pond space is too valuable to waste on koi that will not gain value.
So then we are left with the 2000 koi they decided had potential to become more valuable. The next season they pull them up and sell maybe 1800 of them as Nisai (two years old). Again these are koi they feel have reached their full potential and will not be worth more later on, so they are sold. But remember, these koi were all higher quality then the rest and represent 2% of the entire spawn and two seasons of hard work. So they sell for a much higher price. Usually these will be the koi in the $200 to $800 range but some of the best will be sold for much more.
However the 200 that are kept another year are the absolute very best and represent .2% of the original spawn. They are grown for yet another year and perhaps 180 of them are sold as Sansai (three years old) at a much higher price, usually several thousand dollars. Then a year later the last 20 are left and perhaps 18 of those are sold for an unbelievable amount of money. The best one or two are usually entered in the All Japan Koi Show and frequently held for future breeding.
So as you can see, it is not likely that one of those first koi that sold for around $20 could ever be worth as much as those that were held by the breeder for years. Those large koi being sold for the big bucks not only had a lot of potential as a young koi, but were also raised in the superior environment of the breeders mud ponds. These ponds are ideal for koi and allows for maximum growth and color development. Koi that spent their lives in a small backyard pond simply can't compare. Sure every now and then a lucky little koi slips through the breeders culling and finds it's way into a superior hobbyist pond and they end up with a $2000 koi years later, but that doesn't happen very often. Odds are, if the fish holds its pattern and develops a nice body, it will be worth about what you paid for it even though it is 10x bigger. Sorry, but just because you have a bunch of large koi that doesn't mean you are sitting on a retirement goldmine. If you want a high quality valuable koi, then you will need to buy one.