A lot of the time when I want to sell fish, I'll measure them when they come up to the glass, but when I actually pull them out and measure them again, they're usually about 1/2" bigger or more. I also think that my fish look small in my tank, but when I take them to a lfs to trade in and they put it in their tanks, I think they look pretty big
And friends will come over and comment on how big my fish are and I don't really notice it.