Are larger fish slower than smaller ones of the same species?

Hi,

just wondering... would a 10cm long oscar be a faster or slower swimmer
than a 5cm long oscar? You can probably guess why I'm asking...

I would have thought a smaller fish would be a faster swimmer than a
larger one, but then the larger one has more power. And it seems as
though my larger oscar can still catch up to the small one in order to
take a nip of his tail occasionally. Might head off to BigW to buy
another hiding place for the little one...