The 10 percent rule of thumb remains the "gold standard" as a starting point FOR MOST PILGRIMS. It is a generalization. However, many, many pilgrims find that it does work.
All pilgrims, and their gear needs, are different. Thus, their loads, and the packs they use must also be tuned individually to their personal needs. Dougfitz and others have sought over the years to develop more or less objective criteria. It is a worthwhile effort, but the target constantly moves.
I started using a 48L Osprey Kestrel Rucksack, and used it for two month-long
Camino Frances, and one Camino Portuguese. However, coming off my third Camino last April, by the time I reached Santiago from Porto, I had experimented by mailing packages of clothing and other gear I triaged regularly, ahead to Ivar. I typically sent a box every three days. By the time I reached Santiago, my 48-liter rucksack was only about 2/3 full. The carried weight decreased from 13 Kg at the airplane check-in, to 9 Kg on arrival at Santiago.
So, this coming April, I will walk the Camino de Madrid using a 35-liter Elementerre "Arenal" (FR) Rucksack, with accessory front Ribz Packs. The accessory rig is intended to move water, rain gear, and anything else I might need during the walking day to the front of my harness to balance weight. This bit is an experiment that I will report on here in the Forum, after my Camino. This new rucksack has one main compartment, and will hold everything I do not need until I arrive at my lodging for the evening. That should keep managing the gear easy.
I decided on this "zoned gear" arrangement after studying everyone else I encountered, and carefully evaluating everything I have brought over three Caminos, and plan to bring this year. I also lurked continuously in the Forum, "cherry-picking" the best ideas I read. I maintain an Excel worksheet that weighs EVERY pocket's contents and aggregates the weights in grams and ounces, I use a dietary scale with a plate-sized flat surface. Each year, I make a copy of the previous year's worksheet and start carving away, based on the notes I took along the way.
After four years' of experimenting and evaluation, I have MY personal system just about perfected. But, and as I stated earlier, IT IS MY SOLUTION.
The good news is that I actually have it down to 11.3 Kg, packed or worn weight. That is about 10 percent of my naked weight; I am a stout fellow...
However, if I use the "From the Skin Out" approach (FSO), the total rises to 15.8 Kg. That includes all worn clothing and footwear (boots), as well as anything in any cargo pocket or attached to my trouser belt. Yes, it includes all electronics, personal items, medications, etc. I even weighed my pocket change...
I am aware that some may consider this a tad heavy. I agree. However, the weight does not feel heavy to me, and I have carried far more in the past. Plus, I have a built-in handicap...
As a consequence of chronic health issues, and creeping, advancing age with all it's aches and pains, I must carry extra medicines, dietary supplements, knee braces, and related paraphernalia. The extra items weigh about 2 Kg at the beginning of a 30-days journey.
I also use a mid-route, prepositioned cache to ship ahead dietary supplements, and other medications that I consume each day. That means I start with two-weeks worth of supplies, and send the other two-weeks supply ahead. Otherwise, the extra load for a 30-day Camino would be 4 Kg, instead of 2 Kg. I suffered that on my first Camino. It caused me to get real clever and creative, FAST!
As an aside, I have seen both packing extremes on my Caminos.
On my first Camino, I encountered a fellow from one of the Eastern European countries who was traipsing all over Europe with his "house' on his pack. He was carrying a military specification, 80-liter Snugpak rucksack with internal frame. Strapped to the rucksack at appropriate points were a tent (with poles), foam sleeping pad, and a rather large looking sleeping bag in a stuff sack. Otherwise attached to the exterior of the HUGE olive green rucksack were pots and pans, hats, extra boots, and a guitar (full size Spanish guitar). The lad carrying this literally leaned forward under the load as he walked. It cannot have been comfortable.
On the other extreme, on my second Camino, in 2014, I met a team of four people, two couples, who were RUNNING the entire
Camino Frances - BACKWARDS, from Santiago to St. Jean Pied de Port. Sheesh! It takes all kinds. Mine is NOT to judge...
Anyway, I did speak to these folks for a brief while. None had a backpack, and they claimed to not be shipping rucksacks or luggage ahead daily. Each person was wearing tafetta-weight running shorts, a t-shirt, and low cut socks with trail runner shoes. Each wore a lightweight mesh ball cap.
The only items carried / worn were a waist belt with two .5 liter water bottles and a small zip pouch containing passports and wallets. One person carried a shared first aid kit, another carried a supply of electrolyte powder. When someone else asked what they used for a towel, each produced a Buff. Also, I recall that each person had a super lightweight runner's jacket in a bright color that would have been wind and rain resistant in my assessment. The jackets may or may not have had an attached hood.
But, I would gauge that these folks were wearing carrying not more than 3 Kg FSO.
So, there you have it. Just as your camino is YOUR camino, your packing solution is YOUR packing solution. I opine, based on experience and observation, that a normal sized person would most likely need a 30- 40 liter rucksack with an internal frame.
I HAVE seen many pilgrims with 25 to 30 liter-rucksacks. However on questioning, or direct observation, many of these folks had shipped luggage or larger rucksack ahead via mochila transport services. They were able to wear day-pack sized rucksacks as they had their clothing and toiletries, etc. waiting for them, at their night's lodging.
Then again, and as I mentioned above, it IS possible to go ultra light. If you used all state of the art gear and clothing to reduce size and weight, you could do it. But, that avoids using what you have already.
Bottom line, 55-liter rucksack is too large IF FILLED. The 25-liter pack is likely too small, unless used with all state-of the art gear to reduce size and weight. Also, and as others have correctly stated, any pack should have a padded hip belt, and a sternum strap. Sternum straps can be added to many bags. However, padded hip belts typically cannot.
I hope this helps...