>full of cows
Better kill the one who uploaded the damn thing to sukebei then.
>trying to correct grammar when you don't understand it yourself
What is the sentence's subject? "Era".
What is the era doing? "Starting".
The era is starting. The big girls are a quality of the era, so they're also starting, but they're a post-modifier for the subject, not the subject itself.
Also, notice how unnatural "big girls are starting" sounds. As "girls" are not a period of time like "era", you'd need to explain what the big girls are starting to do. If "big girls" were the subject, the sentence would need to be something like: "Big girls are starting to become more common in erotic manga."
The pronunciation of おう" isn't even similar to "oo", as in "root", or "ou", as in "pout".
If you want a phonetically correct spelling, use ō. "Tōsaka".
Japanese romanization has nothing to do with English pronunciation.
おう may be spelled out "ou", "oh", or "ō", but it's always pronounced the same long o as in "Oh my god, what the fuck am I doing explaining this shit to some retard on the internet?"