I guess fish are like people, and all fish don't grow the same size. How often do you do water changes in that tank and what size is the tank? Not sure about the silver dollar species, but some fish actually put out a hormone that can stunt the growth of other fish, and the only way to keep this down is water changes.
Thanks. I change the water once a fortnight. The tank I just had them in was 30cm to 30cm (nobody told me in the pet store just how big these fish get!!, and I moved them last night to a width 30cm by height 50cm tank so they are definitely happier. The big one is now 7cms and the other one is still the same size as when I bought it 3-4cms.(its a little fatter though).
Any recommendations of fish (that won't get any bigger) to put in my empty tank, I'd rather not ask the petstore again incase they sell me a shark or something. I'm already guessing I will need a bigger tank for the Silver fish in 3mths time.
Thanks for your help! This fish stuff is very complicated but additive.