When shopping for a backpack, the voice of common sense says to make sure you get one big enough to carry all your stuff. And, when in doubt, size up for good measure. Obviously your backpack needs to hold everything you're taking with you. The problem is a big pack also encourages more stuff. More stuff equals more weight to lug down the trail. Nature abhors a vacuum, and empty space inevitably fills with "just in case" stuff. This leads naturally and relentlessly to a heavier load. More stuff equals more weight.
Below is a summary of my own backpack evolution from big and heavy to small and light. Now that my typical load is 19 or 20 lbs., it's hard to believe I used to carry as much as 44 lbs. Lighter is more fun, believe me.
2005 – Gregory Palisade (6 lbs, 14 oz.) 4800 cu.in.
2009 – Granite Gear Vapor Trail (2 lbs 7 oz) 3600 cu.in.
2015 – Mountain Laurel Designs Prophet (1 lb, 1 oz.) 2900 cu.in.
A large pack weighs more (sometimes a lot more), even when it's empty. A smaller pack weighs less and holds less, so you carry less. It's fairly simple.