I usually use a 46 liter Osprey which is supposedly carry-on size, certainly seems it, and I've never had an issue. It's a bit easier to squeeze into tight overhead space if it's not too jam packed full.

I use the same. I'm not sure it will technically fit into the baggage check thingy at the gate if it's packed full but I've never had to check it unless the overhead bins were already full or on a short hop flight with smaller overhead bins where most bags get gate checked.

I have 40L/50+10Ls that I can even fit underneath the seat

Even packed to the gills, I've never had an issue on any domestic or international flight using it as a carry on. I can recall maybe one or two occasions where no overhead space was available and I needed to remove a few items from the fully loaded ruck to fit it beneath the seat, but those were situations were outliers.

The 34L that I have fits under a plane seat even if its packed full. I prefer it in overhead obviously, but for a short flight with packed overhead, it's nice to have the option.