This will be difficult because it was only roughly 5 seconds worth of content.
I'm looking for a show that featured a nighttime cityscape backdrop and an anime transformation sequence which ended with the girl's chest size growing exponentially. I believe she was wearing red, but other than your typical Sailor Moon type style of song and spinning that's the only thing than I can remember because right after the boobs thing, my mom walked in the room and I had to slam the change channels button.
But it's haunted me for years.
It was a re run of an old show. Very late at night, it was in English on a channel that did not typically have cartoons