I put my Tristar on top of my U116 all the time. I put my Aeronaut there, too, when I travel with it. With a U116 and an Aeronaut or Tristar, I can pack enough for several weeks if need be, yet still never have go check luggage, even if my flights include legs on the smalles CRJs. That's the whole point. The U116 will fit any sizer, and has 3 big advantages over most other similarly sized rolling totes:

1. it has a single main compartment, which makes it more flexible for packing for a variety of different types of trips. I use packing folders and cubes to organize within that main compartment. Which types of folders and cubes I use depends on what stuff I'm packing, which can vary greatly between a vacation beach trip vs a business meeting.

2. It has a top-loading main compartment (as opposed to front loading). For me, this makes it much more convenient to travel with, since I can reach in from the top to pull out a single item if need be, without the other contents spilling out the front.

3. The top of the U116 is basically flat - it has no slope to it. This is huge for me. The slope-topped rolling totes are a huge pain to balance another bag on top of, since they always want to slide off.

Now, I grant you that when I put a larger non-wheeled bag that doesn't have a built in handle pass-thru on top of the small U116 (Tristar, Aeronaut, Skytrain, etc), it doesn't always want to stay put. My simple solution can be seen in this post.