Enforcement of carry-on bag size varies greatly, depending on staff. Usually they are concerned with getting the flight out on time first and foremost and don't bother - but sometimes they do bother and then you basically have no choice except to pay up (especially when the sizer comes out not at check-in but at or even past the boarding gate when it's too late to do anything but pay) .
My advice would be to not buy new smaller bags but simply not use the full capacity of your existing backpacks so they'll comply with the size requirements with a bit of squishing and fit through the sizer when asked to put through at check-in. That has worked for me on a great many trips on Ryanair-style airlines all over the world without ever getting into trouble (not on Ryanair itself, that company is on my "avoid at all cost" list ever since they pulled a O'Leary hissy fit stunt in 2008 by leaving my home airport over a "I want you to lower fees for me and if you don't agree, well" dispute with no notice leaving employees and customers out in the cold". That cost me more money than anything ever saved with their admittedly cheaper-than BA-and Easyjet tickets)
Ryanair got into some hot water with Spanish authorities a year or two ago with really bad press and a public outrage at enforcement of their policy of including airport-bought food in their minimal hand-baggage allowance and charging excessive fees for bringing it on board (Next Level: Ryanair Tried To Charge Passengers $100 To Bring 2 Cakes Onboard) and enforcement has been scaled down as a result. The airlines have zero desire to see these practices challenged by legislation or high court decisions, which could well happen in the EU.