As Paul says
you would need some bigger birds first you would be very highly unlikely to breed large birds from two small birds
pair your best birds together best cock to best hen 2nd best to 2nd best ect...
then by selecting only the best birds and releasing others in your stud after a couple of years you should start to see a uniformity in your birds
by pairing your best birds to lesser birds you will dilute the quality
i would add by choosing penguins you have chosen a colour that tends to be smaller than others so judge them against other penguins although they have improved a bit over last few years
dave