It all depends on the actual processor being used. If you buy a laptop that is using a standard P4, then, yes, it will take up a good bit of power. However, more modern processors, such as the mobile Core 2 Duo, are manufactured with battery usage in mind. There IS a difference between the notebook and desktop processors, even though they may have the same generic name (i.e. the Core 2 Duo in your desktop (Yonah) is different from the Core 2 Duo in your notebook (Merom)).
As for battery life being shorter because programs are "bigger," you also have to consider that the hardware has scaled somewhat accordingly. One of the biggest drains on your battery life will be disk usage. Having a notebook with a beefy L2 cache and plenty of RAM to prevent page outs will help you get more time on the battery.
You really have to look at what the notebook manufacturer touts its product as. Those meaning to be desktop replacements will sacrifice battery life for big, beautiful displays while those aimed at being mobile solutions will sacrifice a few points on the benchmarks to get you long lasting work time. Take, for example, the latest T-series from Lenovo. They are fairly powerful, yet can still net you over six hours with the extended battery (around 4 on the standard). The newest 17" Mac Book Pro (which is gorgeous) advertises up to 5.5 hours on battery. Real world usage will be lower, obviously, but nowhere near as low as 1.5 hours. Any laptop that gets you under 2 hours of battery life off the shelf had better be running an impressive display or some serious hardware under the hood these days.