[sfeng]

For one, developing reliable rocket engines takes a long time, you can't just whip up a new one. The current Merlin has a very large throttling range already when compared to existing engines. The size of the engines they use now is defined by the need to get payload's to space, the landing has to be secondary.

[simonh]

Grasshopper was able to hover, or even perform a powered descent and was probably lighter than an F9 first stage, so I don't think this can't be it. They likely do have an engine configuration capable fo a lower minimum thrust, but they're choosing not to use it presumably because it's not optimal for the primary mission, and they believe it's not necessary for the landing.