It sounds like you aren't camping-just traveling with a backpack. Big difference in what you will be taking. Without camping gear, you don't need as big a pack.

40 l. may still be a bit small even if you aren't taking camping gear, but the best way to tell is to gather up all your gear, go to REI or other store with a good selection and see what pack holds it all. Save some room for souvenirs. If all you are taking is clothes, then 40 l. might work, but will generally be too small if you do any camping later.

Lighter doesn't necessarily mean smaller. A liter = 61 c.i. so a 60l pack is about 3660 c.i. and 40l = 2440 c.i. more or less. There are many packs in this size range and they will vary widely in weight depending on the maker, features and price.

Granite Gear makes some nice, light packs. More money than the North Face packs and REI may not have the whole line, but worth looking at. REI has its own house brand of light packs.

Take a look at the packs offered through the site sponsors (click on the Portal link at the top of the page). This should give you a good idea as to what is available.

I would stay away from the UL packs-some of them are very light, but I question whether or not they will stand up going through an airline's baggage handling system. By UL I mean the packs that weigh a pound or so and are basically a rucksack made of very light material.