Sorry you're right, I added that 'no one uses it' comment on afterwards in reference to the Mega- prefix and in my exhausted state didn't parse the milli- versi

Sorry you're right, I added that 'no one uses it' comment on afterwards in reference to the Mega- prefix and in my exhausted state didn't parse the milli- version which indeed people do use. Per my latter comment, it's because we don't really deal with everyday words for masses smaller than a gram, but we do for those bigger. It still holds that as far as I can think, people will go to the closest 'common term' word and then use prefixes to bridge the remaining gap (milligram, megatonne)