Hmm. Yes there are many REI clerks who advise as you describe. Traditional backpackers are certainly the norm, and REI clerks are no different.
I am a part-time REI salesclerk. To the extent we can, we advise from our own experience. It's not that REI is deliberately training us to sell bulky gear. People want certain features, and those features have weight consequences. People come in wanting huge packs for multi-day trips. At least I can explain that adding days does not need to add volume in some linear calculation; generally they assume it does.
As to the "source," the best I seem to be able to do with folks is explain my own experience. As in, here is how I do it. I tell them I like tarps. Or alcohol stoves. Or the higher end down sleeping bags. Or closed-cell foam. I'll hand them a Flash 65 pack and, for example, and Aether 60, so they can compare the pack weight directly. After a weighted try-on, though, the advice-seeking customer much more frequently will choose the heavier pack. (We sell a ton of the Flash 65 and 50 packs, but mostly to people who come in, grab 'em, and check out, without asking for help. They already know what they want.) But my personal experience comes in a context: I am not very big, and certainly not young. So I tell people about specific trips. Like having to keep my kit weight down so I can haul 16 lbs. of water up for 2 days in the Guadalupe Mts. Sometimes works, often not. I am forever telling parents that their 13-year-old does *not* need his own solo tent! That he (it is nearly always a boy, why aren't parents outfitting their daughters?) needs to share. Sometimes they agree with me, mostly they demur.
I don't get the impression that folks are discounting my recommendations because I say I prefer going light, but rather, it seems that they have a fear of anticipated discomfort, think they can alleviate discomfort with more or cushier stuff, and fail to see that more stuff can create discomfort. For newbies, normally there is no talking them out of this--they need experience.