Why would anyone ever say an 18g is too small? Too small for what... It depends on what you need it to do.
Damn Boost Junkies.
An 18g is great for a lot of different stuff like daily street driving and autoXing. A 20g won't be as good for those but will build more boost up top. Maybe bigger boost was their basis for comparison. There is a purpose for every turbo and there isn't one turbo that is best at everything. Enjoy your choice and move on.