When I saw the "65L" bag, I gasped, because that's 70% larger (!) than the bag I used for my last RS tour (summer 2022 Best of Ireland in 14 Days) plus 3 weeks of self-planned travel before and after.
Having never owned a bag that big, I looked it up. According to REI (where it's now on sale) its dimensions are 26 x 15 x 10 inches and its weight (empty) is 6 lbs. 13 oz. I can't imagine schlepping that huge bag around on an RS tour, or any kind of European trip at all, much less fully packed.
I do carry-on only and limit my bag to 20 pounds fully packed. I'd do that even if I checked my bag because I travel solo, whether on an RS tour or on my own. Anything bigger is not only more awkward but also going to be much heavier fully packed.
I typically travel for 5 weeks at a time, with a 2-week RS tour sandwiched between self-planned time. I have 2 US standard 22 x 14 x 9 inch "large" bags, roughly 42L capacity, one Eagle Creek and one Osprey Ozone. Both weigh less than 5 pounds empty. I haven't used the Osprey yet and I haven't used the Eagle Creek since 2018.
I've managed to downsize my packing enough to have plenty of room in my 2 "international" bags. I've used them on one 5-week trip each so far and both worked well.
My Eagle Creek International 2-wheeled bag is expandable, but I didn't need to expand it. Its capacity is 36L --> 39L, its dimensions are 21 x 14 x 8 inches --> 21 x 14 x 9.5 inches and its empty weight is 4 lbs. 9 oz.
My Osprey Ozone Global 2-wheeled bag has a capacity of 38L. It is not expandable. Its dimensions are 20 x 16 x 8 inches and its empty weight is 4 lbs. 11 oz. Before my last trip I test packed all 4 bags and much to my surprise, this one both held everything and was easiest to pack, largely due to its shape. I was flying British Airways, so the 16" width was not an issue.
As others have said, the RS bag size is highly encouraged because it supports efficient packing and moving around in places where the terrain may be rough, there may be steep inclines and/or stairs, the bus may not be able to get anywhere close to the lodgings and the group may be required to get to and from the bus quickly. I've been on 6 RS tours so far and experienced all those situations. Those who followed the recommendations definitely had an easier time with that than those who didn't.