Prefix equivalents in English?
I don't think there is a universal prefix equivalent to 小 or 阿. Certainly you can add 'Little', but that's not a prefix, but an adjective. 

In the West, nicknames follow different patterns, mainly using the first syllable of the full name and adding an [i] (-ie, -y or similar):

E.g. John --> Johnnie, Andrew ---> Andy, Lisa ---> Lizzy, etc.

Other countries have similar fashions, adding -(e)tte [French], -chen, -le [German, with some regional differences], -ito,-ita [Spanish] -ka, -ška, -čka, -ek, (Czech and other Slavic with different spellings).  

Some nicknames are completely different from the formal given name, e.g. Richard --> Dick.