Dam-/dcm- (NEB #C2925H/C2925I) is a mutant strain of E.coli and the dam mutation affects this strain in a number of ways including lower transformation efficiency, lower growth rate and higher mutation rate. It is normal to get different size colonies for NEB #C2925H/C2925I. This was noticed while transforming pUC19. Both size colonies were tested and they all contained pUC19. A single large colony was picked and stirred in SOB and plated on LB/Amp. Again, both large and small colonies were visible the following morning.