To clarify, it’s not that my wife’s 33L pack wouldn’t fit the overhead carry-on space, it’s that her 33L pack did not meet the stated Iberia dimensions nor the carry-on sizing containers at the airport. These are clearly given on their website:
“Iberia Airlines allows you to travel with hand baggage at no charge, but it has to fulfill the following requirements: The maximum dimension per carry-on bag is 56 x 45 x 25 cm. You must include the handles, compartments, pockets, and wheels when measuring the baggage.”
Apparently not all airports or flight crews strictly enforce this, but when they do you must then purchase “hold luggage” at over a 50% markup from pre purchasing that option prior to checking in. At that point, you would need to remove anything you cannot lose or of value (medications, electronics, etc).
Whether you decide to follow the rules or roll the dice is up to you, but as of March 2022, this was the rule at Iberia (as well as many other low-cost European airlines).