According to the Consumer Product Safety Commission, the Switchback and Trigger Action travel mugs are getting the most complaints. The source of the issue is the lid. As per their report ...

Any mug with a lid that you can carry from place to place is technically a travel mug, but the best travel mugs keep your iced or hot beverage to temperature, won't leak all over your bag ...

And then, of course, 16 ounces is somewhere in between. The type of lid on your travel mug can make a fairly big difference in how you experience drinking. Some have bigger openings, some smaller.