If you are relatively young and strong, an oversized (which usually also means heavier) backpack is only a "problem" in that a lighter pack usually allows you to go more miles if you are hiking long distances. Comfort and being sized for the intended load is more important.

As you get older (there are a few of us here that are 70 yrs old + and still backpacking) smaller and lighter backpacks and lighter gear allows one to backpack well into old age!

I do disagree that you can only use a smaller pack in "warm" weather. Not sure what you mean by "warm". I can still use a 55L pack for weather down to 10F nights and 40F days. For me, I switch to a larger pack if I need more room for more food (more than 10 days),in real winter conditions where I need a lot more clothing, or if I take the grandkids and have to carry a lot of their stuff.