It just depends on the airlines.
Check the website of your airlines and see what size they allow.
Then consider using a service that wraps your pack tightly in saran.
My pack is quite a bit smaller than yours, so I'm not sure how small yours can be made.
The other option is to pack all non-essential items - things you could easily replace - in a checked bag.
Then try to squeeze your pack down smaller using saran?
Annie is correct, of course. But, I cannot help thinking that perhaps your packs are simply too large, especially for a summer Camino. I do not have all the facts, so this is just a supposition on my part.
My personal rucksack for three Caminos was an Osprey Kestrel 48 liter bag with internal frame. It is too large to carry on, plus it contains all sorts of things that I cannot carry on. So, I use an inexpensive mesh laundry bag I bought in a local Dollar Tree store for ONE DOLLAR to check the rucksack, poles, sharps, etc. I simply cinch the opening close with the toggle fastener and attach two luggage tags. One is my home address, the other is my destination for the first evening.
Also, I wear a complete hiking outfit including my boots on the flight over. I carry-on my documents, medications, rain parka, a zip-up 100 level Polartec fleece, a hat, and a change of shirt and underwear in a 20-liter sil-nylon backpack that compresses to the size of a tennis ball. If my checked rucksack gets mis-directed, I have enough to tide me over. So far, I have had no problems. But I am a firm believer in the adage that an ounce of prevention is worth a pound of cure...
As mentioned, I attach a "destination tag" to my checked rucksack, showing my first night's destination (hotel usually so the airline knows where to send the bag. This tag includes my e-mail and European phone number (I maintain a pre-paid Belgian number as I travel to Europe several times each year.).
I should note in passing that, on my most recent Camino, in April, I mailed so much stuff ahead to Ivar that my 48-liter rucksack was only about 2/3 full by the time I reached Santiago. The next time, I am going to give a 35-liter bag a try. THAT bag will be carry-on size. I will still have to check my hiking sticks and sharps. But, this will lower the net risk.
I hope this helps.