Is it just the one eyeball that's big?
If it is just one, the most likely cause is injury. This will heal by itself, though there is the risk if infection getting in. Usually, extra water changes to keep the water extra clean (to get the fish poo off the bottom) is all that's needed. But watch for infections starting.
When it's both eyes, that is usually a sign of an internal infection. This is much more difficult to treat especially in the UK as we don't have access to antibiotics and have to rely on such things as Interpet anit-internal bacteria or possibly eSHa 2000.
If your dwarf rainbowfish are anything like mine, injury is a distinct possibility the way they dash around.
