Do not, repeat, DO NOT follow some random stranger on the internet or anywhere else who says you only need X number of liters. That person might be 5' and 90 lbs. If you're significantly larger, the advice that you can get away with a small pack will leave your pack bursting at the seams and you possibly ending your Camino early due to an ill-fitting pack.
If you're a bigger person, you might need to get up to the 45l-50l range before you find a pack that is actually comfortable to wear and can hold your clothes. Let me use myself for example. I'm 6'3'' and 300 lbs. I am not small person. When I go on an overnight trip, or anywhere that I need to bring an extra set of clothes, I have a regular old book-bag style backpack I use. the capacity is about 30 liters. I can fit a shirt, a pair of pants, sweatshirt, underwear and socks, and some basic toiletries in it. Things like a sleeping bag, rain jacket, water, a pair or sandals or sneakers, or any other gear that one might want on the Camino... just not happening. And even with just the regular amount of stuff I have in that bag, I would not want to walk 24-30 kilometers a day with it on. It would kill my back.
I remember talking to a petite woman on the Camino and were discussing packs. I told her the clothes I had packed, and she said it was about the same as she brought, so why did I need such a big pack? Well, one of my t-shirts had enough material to make 2 or 3 of hers. So multiply that over a couple of shirts, pants, socks, etc. and I just have more material to deal with. More material means more space and more weight, so a bigger bag is necessary.
Also, the time of year you walk is a factor as well. Someone in July and August has a much different pack list from someone walking in March. Long sleeves and pants and insulated jackets vs. shorts and t-shirts and thin rain jackets.
The best advice for choosing a pack is to go to a real outdoor store and get your torso measured. Try on a few different packs. Get a feel for them, what's comfortable, what's not. What has the features you're looking for. (Most outdoor stores should be able to load the pack up with weight so you can get a real feel for it.) But don't buy that day. See what packs you like, go home and do your research. Read the reviews. Unless the pack is made by some small boutique outfit, there's probably a dozen youtube reviews for it.
The 10% rule is okay for a basic guide. But don't kill yourself to reach it. If you're carrying 15% and it works for you, go for it!