I have both the 25 and the 19.
I am 5'10 and I find that with either bag I prefer not filling them more than about 3/4 full.
I use the 19 for very short hikes, walking the dogs then needing a place to put my sweater when it gets warm, walking around museums when I don't want to use my copilot off to side, etc.
For a plane trip, a medium hike or a full day out touring around a European city I definitely use my S25.
On plane rides I've gotten to use my "toms slippers" after I take off my shoes and walk around, when I'm ready to land I slide a thin sock around each one and slip them on either side of my "e bags slim lunch box. Misc items r distributed between the pockets and my snake charmer which fits into the bottom section.
At times, such as in a museum that doesn't allow backpacks around the galleries I have at the ready my medium cafe bag or steel dyneema copilot ( which sits wonderfully at the bottom of the main compartment bring used as a packing cube.) which then comes out as I check in the S25.
One side note, when I'm doing this "nesting" thing with the CoPilot in the S25 I find that the standard shoulder strap is more manageable as a stored item than my absolute strap.
I could ramble on, but I've got to get back to work.