Ochu is just Ochu. Microchu, Nanochu, and Picochu are pretty obvious. I can't figure out Chichu though. All I can think of is Chi is a Greek letter which has two different pronunciations depending on if you're legitimately Greek or not Greek. So what does the name mean and is there also a consensus on the proper pronunciation?
English names can be deceptive, since they often do not reflect the intent of the Japanese names if translated improperly or with too much liberty. This is just such a case.
The Japanese names for the little 'chu's do not imply any sort of metric system nano-, micro-, pico- references. The Japanese names are as follows:
Microchu in Japanese is Kochu (ko here means a child. Like with the Japanese names for Chicobo and MiniMog from FFVIII - kochocobo and komoguri)
Nanochu in Japanese is Pikochu (Nanochu in FFXIII-2 is actually supposed to be the Picochu from FFXIII, but they messed this all up when translating). It's possible the piko part could refer to the pico- prefix, but I find it unlikely, since none of the other chu's follow this pattern in Japanese. Probably just a play off of kochu.
Chichu in Japanese is Kichu (probably just another play on kochu, but ki has various meanings in Japanese, any one of which could be applied here.)
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I always liked to think of "Chi" in this case as being a shortened form of "Chisai" (Japanese for "small"), making Chichu = "Small Chu". ; ) It fits right? Only makes him even cuter. :P Also, "Chu" in Japanese is the onomatopoeia for the sound made when kissing, so I guess you could stretch it and suggest the name means "Small Kiss". : D
chichu is definitely smaller than ochu but I think he's still pretty big with respect to microchu and nanochu (I could be wrong), but also with Espritduo's claim that chichu is kichu in Japanese (with chi and ki being different characters), I don't think they had the intention of his name to mean small ochu