If you will potentially need that extra capacity on a future trip, then definitely get the 75L pack. If you're doing it just because it doesn't seem like much extra weight, even if you don't expect to ever need it, I would go for the 65L pack. That would also curb the temptation to overpack.
70L is definitely too big. I believe the 55L is a 40L + a 15L daypack. This will probably work, but double check the length, width, and height, and make sure none of those dimensions are larger than allowed for a carry on.
Trips of 5 days or more usually call for packs of 70 liters or more. These are also usually the preferred choice for winter treks lasting more than 1 night. (Larger packs can more comfortably accommodate extra clothing, a warmer sleeping bag and a 4-season tent, which typically includes extra poles.)
What happens if your backpack is too big? An overweight backpack causes the body to compensate for the extra weight. If you lean forward to compensate, it reduces your balance making it easier to fall. The extra weight can distort the natural curve of the middle and lower back, causing muscle strain.
The weight of backpacks that seals carry on missions can vary depending on the specific mission and equipment required. It depends on the mission. If it's a quick in and out or if they have vehicles their packs might weight about 20-40lbs depending on their position in the team.
In most cases, the compartment size may range from about 35 to 45 liters. In some cases, the bag may also have an expandable pull string storage for more space.
Your backpack is too heavy if:It's difficult to put on or take off. You have pain from wearing it. You feel tingling or numbness in your limbs. Strap marks show on your shoulders.
Is 65L backpack too big for Europe? I recommend a backpack around 40L-50L. Personally, I wouldn't go over 65L, but some people like a bigger bag. You can always go smaller, but I wouldn't recommend anything smaller than 35L — unless you're into minimalist travel.