It all depends on what metrics you use, but if support is anything to go by, the Top 14 is definitely bigger (even the relegated second division often has higher crowds than the NRL), and the Premiership is probably comparable. On an equity POV however, both, plus the Top League in Japan are undoubtedly bigger (where companies and billionaires stump up millions).
Also, it should be noted that the European clubs (which are actually clubs like the NRL... not provinces like Super Rugby) compete in two different leagues in one season: both domestic leagues and the Champions Cup (Heineken Cup) to determine the best of all the leagues in Europe.
The domestic leagues (Top 14, English Premiership, PRO14) only make up half of the club season. It's not quite fair to compare NRL and domestic European leagues because of this, but if you factor the entire club season of a European club, it is certainly bigger. Also worth noting that the 6 Nations is one of the biggest annual sporting fixtures on the planet. It is MASSIVE!!! (and that is the level EJ is operating at).