Is 75L pack too big?
Multiday Packs: 50-75 liters
Packs specifically for expedition or winter camping will be on the larger side (over 75 liters) to accommodate a warmer sleeping setup, mountaineering equipment, and first aid and survival gear.
Is a 75L backpack a carry-on?
A 75 liter luggage is considered somewhat large to be carried into the aircraft cabin and it will most likely have to be checked in.
Is 70L too big for backpacking?
I would say though, in general, 55 to 70L is a good range to be in. Most good packs are very light so a little extra room at 200/odd grams more won't hurt you. But also very very important is to actually try on many packs.