Generally, I'd agree that it's better to cinch up a too-large pack than try to overload a too-small pack. However, it's not because of the size of the packbag.

If you look at the specs for most packs, you'll see they give some variant of a "comfort range" that specifies how much weight the pack can carry and still be "comfortable" (for an "average" person, whoever that is.) It's not a perfect, objective system, and not precisely comparable from one company to the next, but it's a starting place.

The larger packs typically have higher load ratings. Yes, the bags are larger - but more critically, the suspension is usually beefier, and it's the suspension that determines the weight-bearing capacity. (In fact, there are some pack frames sold without a pack bag, for carrying heavy loads like firefighting tools, wood, or for hunting. They are all suspension.) Oftentimes, within a line (say, the Osprey Kestrel with which I'm familiar) the suspension design and features will be the same for the entire series, but as the load ratings increase, the hipbelt might be a bit wider or stiffer, and the framesheet might be a bit less flexible or be reinforced by a stay.

So, as you look at larger packs, pay attention mostly to the suspension - especially if you start looking at packs from other manufacturers and other lines from the same manufacturer. I know, from personal experience, that 50 liter packs from Gregory, Granite Gear, Deuter, and Osprey all carry differently with the same gear in the packbag. Within the Osprey line, the Talon, Atmos, Kestrel, Exos, and Aether each carry a bit differently from the other with the same load; each has a different comfort range, and each has a different type of suspension.

So, when shopping for a new pack, it's all about the suspension.