The Ironback has an expansion zipper that runs around the main body of the pack.
Closed: 39 liters. Slim front-to-back. A daypack. Work bag, gym bag, carry-on that fits the overhead bin, the thing you grab for a morning in the woods.
Open: 64 liters. Nearly double the depth. A legitimate multi-day load — sleeping bag, layers, food, water, the whole kit.
Here's the part that separates this from every "expandable" pack that's disappointed you: it compresses genuinely flat again. Most big packs will cinch down smaller but they still ride like a big empty bag with the air let out — tall, floppy, swinging. The Ironback goes back to being a normal-sized pack that holds its shape.
That's the whole idea. Not a big pack that shrinks a little. Two real bags in one shell.