I have occasionally used a frame bag about 3 inches thick. But I think the only people that would have trouble are those that think they are riding in a time trial and are trying to get more aero than the rest of us.
Total volume of those frame bags is not that high unless you get one that fills the whole triangle, and then you lose your water bottles so you then need some of that frame bag volume for water.
I am guessing my frame bag is about 5 liters, and I can keep a one liter bottle inside the triangle next to the bag. The bottle in the photo is not a full liter but I can put a full liter in that spot.
