No, I don’t work for Amazon and that wasn’t an ad but a concrete example of a particular form factor of an underseat bag that came to mind because I happen to own one, and thought it was something other frequent travellers might consider adopting. (I’ve only recently converged on the form factor after having tried many others)

They do fit in most rows except aisle, but even then, because of their small size, they can usually be made to fit in a nearly full overhead compartment (source: recent personal experience). I’ve just returned from traveling around Europe and I can’t count the number of times I’ve managed to avoid inconvenient situations because I didn’t bring a full size rollabord. I’ve even been able to put mine on my lap on a full bus in Spain.