The Mizuho ads I've seen in the WSJ print edition over the past 9-months or so make sense now.
A quick look at Japanese banks in order of market capitalization size [latest share price in yen, in US$, yield, ttm P/E, market cap]:
1. Mitsubishi UFJ Fin. Grp. (MTU) (Tokyo: 8306) 1,530,000 yen (US$13,660), 0.46%, 16.41 ttm P/E, US$141b market cap
2. Mizuho Fin. Grp. (Tokyo: 8411) 910,000 yen (US$8,125), 0.44%, 16.50 ttm P/E, US$97.5b market cap
3. Sumitomo Mitsui Fin. Grp. (Tokyo: 8316) 1,160,000 yen (US$10,357), 0.26%, 12.24 ttm P/E, US$77b market cap
*Data as of 05/24/06 Tokyo close; US$ share price and market cap converted at Y112/US$1; Source: Yahoo! Japan Finance
2-year chart comparing the 3 mega-banks: