Of those three, only Augustulus is a diminutive. Romulus Augustulus (both his names are actually diminutive in form) is so-called because he was still a boy when he abdicated. The other two are derived forms but are not diminutives. Justinian is a derived cognomen from Justinus, and the -ianus ending of Latin cognomina is generally indicative of adoption (c.f. Aemilianus or Octavianus). There's nothing especially diminutive about the ending, which is an adjectival ending meaning something like "pertaining to" (e.g. operae Clodianae). Justinian was adopted by his uncle Justin, and thus adopted the name Justinianus. Maximinus is also a derived form, from Maximus, but again it's not a diminutive, it's roughly the same thing as the -ianus ending. It's the same ending present Constantine's (derived from Constantius) and Antoninus Pius' (derived from Antonius) names. I'm a little fuzzier on this one since it's not especially common until later in the Principate when naming conventions start to change drastically, but it was originally used more or less as a filial form of a parental cognomen. The gens Aemilia's oldest branch, for example, was the Aemilii Mamerci, from the gens' ancestor Mamercus. There was a filial branch of the family called the Aemilii Mamercini but sons sometimes took the Mamercine name even though their fathers were named Mamercus. In the case of Maximinus I'm not really sure where it derives (he was related to Galerius, who held the title Maximianus, so that might be the connection), but for example Constantine's name shows the old filial use of the ending, as he was the son of Constantius