I can't believe I'm doing this, but
here is a link to an international bra size converter. Japanese 70D is US 32C and British 32D. I thought the staff might have made a mistake, sizing a European bra in the Japanese way, but it turns out that the EU and Japan use the same system.
And I really can't believe I'm doing
this, but to see what the numbers mean in real life, Google "Japanese bra sizes explained." From japanprobe.com. All in the interests of further knowledge. Possibly not quite work-safe.
Here is a helpful forum answer about how Japanese sizes are calculated. And
here is an article on changing sizes in Japan.