If you look at the figures over the years, LIRR has done a pretty good job of increasing (and holding onto) ridership it's just that Metro-North has had astonishing growth.
LIRR (and Long Island) have been recovering much more slowly from the recession than MNR, Westchester and Connecticut. The LIRR ridership numbers have closely followed the employment trend in the region whereas Metro-North's ridership numbers recovered above
the trend. (That's from the MTA's own analysis.)
But enjoy it while you can, MNRers. The last quarter of 2011 LIRR ridership grew 2.2% over the last quarter of 2010. That one million rider lead may disappear fast.