To me, its less about sump volume, and more about ease of moving it around, taking it out to clean it and all that. I throw Kalk in there, alk, ect. in there and sometimes it builds up in the bottom.
I have a seperate container for my top-off. It is underneath my tank, and I had it built to my specs. It fits tight under there, and can be taken out to be cleaned or whatever I want to do with it. That is definitely an advantage.
I also put a float valve in it and ran my RODI water right to it. When I want to fill it, I turn a valve in the laundry room and it fills it up and shuts off when it is full. For added protection I shut the valve off in the laundry room though.
I would do it the same way again. It is nice to be able to take it out, clean it out, move it around if I am working under there, ect. It also would have been impossible to add it in my sump unless I wanted to decrease the volume in it.
I get my sump stuff made pretty cheap by a local guy here, so that is probably part of the reason I was able to do this without worrying about how much extra or less it would cost.