I agree. With a reasonably dimensioned backpack on your back there is a high chance that no one at the gate will notice and you will be allowed to board without fuzz. The gate agents are looking for trolleys, large carry-ons, 2 pcs of hand luggage etc.

Anyway, if singled out you will be fine as long as you are able to squeeze the backpack into the "sizer"; no one will measure your backpack.