I doubt Coinbase is inflating their volume numbers. They seem really focused on legal compliance, licensing, and working with regulators for stronger oversight and they're trying to push into institutional markets. The last thing they want right now is a scandal around volume manipulation. They seem pretty content staying around #50 on CMC and focusing on real (rather than perceived) market penetration.
Technically speaking, Coinbase is still one of the top exchanges if you discard everything related to USDT.
Top volumes as we speak in actual USD;
CME; $74 million. (contracts, but still, everything is settled with actual USD)
Coinbase Pro; $30 million.
Bitstamp; $19.5 million.
Kraken; $15.5 million.
If Coinbase or any of these exchanges would inflate their volumes, it would be inflated to a level they can compete with the Asian wild west show.