Well, this kind-of gets into a very grey area. What is a a light pack and how does the weights fit into each category: Regular, Light, UL, SUL, XSUL?
In answer to your question, for an internal framed pack I use an old, as in very old, Tough Traveler. This is about 3000ci (50L)and weighs 2#3. It has an internal frame made from magnesium, two side pouches, and a 2" hip belt. I use this most days as a training pack carrying ~50 pounds through the park. I believe it was produced around 1974/5. I don't think anyone currently makes a similar pack, though. None have a magnesium frame. Way back when the kids were little, I would load this with close to 60 pounds of gear and take the family out camping. Really a rugged pack and still in use. But, I don't consider this a UL pack. More of a light weight pack.
Regular packs (heavy weight packs) I consider as anything above 4#.
Light weight packs I consider 2#-4#.
UL packs I consider to be 1#-2#.
SUL packs I consider to be 8oz to 1#.
XSUL packs I consider to be anything less than 8oz.
The current trend in backpacks is heavier, more durability, additional features.
Modular packs are nice, letting a hiker dial in his pack to his load, but this often adds excess weight to the total. Example, your aftermarket side pockets: These are totally enclosed pouches with attachments. If attached to a pack, it is possible to skip one side(using the pack body as a side,) and, only attach five of the six sides and skip the attachment weight for an overall weight savings. Removal, ie, not carrying it, is also a weight savings if you do not need the extra volume. Current philosophy will allow manufacturors to add modular pieces. Strict UL philosophy says no. The lightest pouch should be added. I would suggest not considering any modular packs as Ultra Light, regardless of how much they actually weigh.
Perhaps volume is another consideration, as you mention. I have a couple packs(Osprey, and a wal-mart loaner) that list 50L as the size. There are a couple pouches on the Osprey (for example) that open *internally.* Ie, they subtract from the internal volume when full. They are not big enough for a tent (though they fit a 10x10 tarp, OK.) Simple "organization" pouches like that can be eliminated. They really serve no usefull purpose beyond gear segregation. This is one feature than can easily be eliminated.
With the destruction of most of the early generation cuben packs (for one reason or another) most companies have switched to hybrid materials to increase durability. This was at about the same rate as spinnaker packs were destroyed, forcing manufacturers to look for more durable materials. But instead of .36 or .51 material being used we now find 1.1 and 1.6 material being used. Again, the trend in durable packs is showing. but this means that we now can get better packs even if the weight as increased by 50% or more.