"More likely" might also apply in Japan where it seems all female names end in '-ko'. Yoko, Mitsuko, etc. I know there may be other suffixes but I read of so many ending in 'ko' that it seems like they're all named '-ko'.
Not all, but a lot, yes. The official numbers say it's at the end of about a third of all Japanese female names. It's just a common feminine suffix, like "-ette" in English. "-ko" is more common in Japanese than "-ette" in English, of course, but it's hardly ubiquitous. "-a" is another common feminine ending in English names, because in Latin (and possibly Greek, but I might be wrong here) it almost always denoted feminine gender for a noun. We don't have gendered nouns in English much anymore, though, except for things like boats and ships and sometimes countries.
"-ette" basically has connotations of small and dainty (originally "diminutive"), which was traditionally considered suitable for women, hence it's use in names. "-ko" means "small child", which is pretty similar. It's kanji 子 actually looks rather obviously like a small child with outstretched arms and a trianglely-shape for a head, which is kind of nice, because a lot of kanji are so abstracted or altered that you can't easily rely on them having a similarity to what they're supposed to represent.
Incidentally, "-taro" is a common male suffix in Japanese, and it can even be a whole name by itself. "Yamada Taro" is Japan's "John Smith". I know a few Kentaro, Jotaro, Shintaro, etc.