I may be missing which shoes you are referring to, but are you talking about the ones they call "comfort" shoes like the Wilbert, Badlands, and Bellevue?
If you are talking about these, then I think it's a combination of the foam insole and the "rubber" outsole. I don't believe these have a removable insole (only the orthotics have removable insoles). Rather, they are probably the same insoles present in many other AE shoes right now. Many of their seasonal shoes that are released and then put on clearance after a few months are using the cheaper foam insoles even though they aren't grouping them in the "comfort" category. While their main line (timeless ones that don't really go away) generally retain the nice solid leather insole.
They are a fiberboard insole with a thin layer of foam adhered to it (wrapped with some thin leather for looks). Some descriptions of them say they are Poron, others just say foam. Of course, Poron is just foam, but it's often touted as being better since it was developed by NASA.
I think it's a relative term to consider them lighter, as they may be a few ounces lighter. I don't consider any of my shoes to be "heavy" in other words. A steel shank literally adds a few ounces of overall weight to the shoe, for example. I think most of the weight savings in these models are in the outsoles, which are a very airy rubber. Unlike the harder Dainite soles, they are pretty flimsy and squishy by comparison. The Vibram Gumlite sole is an example of these.
They recently switched the sole on the Long Branch to this Gumlite sole, which I think cheapened the boot significantly, and it made it less attractive due to the thicker profile. But, it probably lightened it up some. I appreciate that they are making it out of Chromexcel now, but the sole was a turn for the worse in my opinion.
I think I may be missing which shoes you are actually referring to though, so feel free to direct me to the ones in question if I've got it wrong.
They recently came out with a line called lightweight comfort shoes - looks like a lot was produced on the 511 so they look like the Wilbert. They are shown on a single page in the 2015 style guide. Trying to find a link to post for you.