Earlier quoted context omitted.
Helen of Troy which owns Kaz, which owns Honeywell (one of the largest businesses specific to Nest's thermostat space) has a market cap of $1.7bil - http://finance.yahoo.com/echarts?s=HELE - that's why this sounds crazy. On the flip side, P&G is a $217bil business - so, not that crazy how big the market is and can be. EDIT: As larrys points out, Honeywell is a $70bil biz.
Sounded odd to me so I checked it out. HoT only appears to have the rights to certain Honeywell products and not the thermostats: http://www.kaz.com/kaz/honeywell/ Edit: "Honeywell" that you are referring to has a market cap of almost 70 billion: http://finance.yahoo.com/q?s=hon&ql=1
On top of that, think of what those guys still have under the hood. Google wants to be the everything of your life. Smartphones fill a part of that but smart devices are another big chunk; and Nest was the company that is undoubtely leading a revolution on the field.
This isn't a strategic move to eventually dominate the thermostat market. This is a strategic move to eventually dominate the 'everything you need' market.